Themeninfo III/2016 A compact guide to energy research

Cooling with solar heat Concepts and technologies for air conditioning buildings

A service from FIZ Karlsruhe GmbH

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Straight to the point The cooling or air conditioning of buildings with solar heat has a particular appeal because the heat demand and supply are usually consistent with each another. Cold stores in southern climates and many process refrigeration systems also require considerable energy when the sun shines intensely. The use of solar cooling systems instead of electric chillers also eases the grid, particularly at peak load times. Solar thermal cooling could develop sales markets, for example, in the Mediterranean region but also play a small part in Germany‘s „Energiewende“ – its energy transition. This is reflected in the research funding provided by the German federal government. In technological terms, German research institutions and SMEs have established a leading international position thanks to their intensive research. However, this young industry sector is facing double competition: on the one hand, well-known companies from the cooling sector, particularly from Asia, are surging onto the international markets; on the other hand, new competition is growing with regard to the technology, since the significant decrease in the cost of photovoltaics has also made solar electric systems with compression chillers increasingly attractive. Most scientists, however, see scope for both technologies. In Europe alone, the need for cooling will quadruple between 1990 and 2020 according to a report for the European Commission. An advantage of solar thermal systems is that they can be flexibly combined with other heat sources. For example, industrial waste heat or energy from cogeneration could also be used. Compared with conventional cooling technology, solar cooling requires high initial investment but has subsequently lower operating costs. It is therefore particularly suitable when the cooling load, solar radiation and price of electricity at the location are high. More research needs to be conducted on the components as well as on the system technology. The following sections provide an overview of the various open and closed methods and provide an insight into the research.

Your BINE editorial team wishes you an enjoyable read

Authors Dr Alexander Morgenstern Dr Mathias Safarik Edo Wiemken Peter Zachmeier Editor Dr Franz Meyer Copyright Text and illustrations from this publication can only be used if permission has been granted by the BINE editorial team. We would be delighted to hear from you. Cover image: Kramer GmbH

Content 3 Solar heat replaces grid power 4 Closed and open methods 8 Applications and system selection

All images are provided by the authors unless otherwise indicated. Lead photos: P. 3 s-power GmbH P. 4 Klingenburg GmbH P. 8 Klingenburg GmbH P. 12 Festo AG Co. KG P. 14 TU-Berlin P. 20 Claus Ableitner (CC-BY-SA 3.0)

9 En passant: Initial attempts with solar cooling 11 Points of view: What are the chances for solar cooling? 12 Planning, cost and integration 14 The research starting points 18 In practice: Absorption cooling for different regions 19 In practice: Success stories 20 Solar power instead of heat 24 Outlook

Kaiserstraße 185-197, 53113 Bonn, Germany Phone +49 228 92379-0 Fax +49 228 92379-29 [email protected] www.bine.info

BINE-Themeninfo III/2016

Solar heat replaces grid power Even in temperate climates, numerous buildings have to be air conditioned. In conference centres, theatres, department stores or high-rise buildings, only indoor air handling units are usually capable of ensuring a comfortable indoor climate. Solar-based methods, however, can particularly lower the electricity needs at peak load times.

Many countries in sunny regions suffer from high loads on the electricity grid for handling cooling and air conditio­ning tasks. In some Mediterranean countries, more than half of the total electricity produced is used for air conditioning buildings in summer. Even a significant ­increase in building standards would not change anything in the short term. The use of solar energy for cooling and air conditioning would seem obvious here, as there is a high correlation between sunlight, ambient heat and the cooling requirement. Solar cooling can effec­tively reduce the electrical energy consumption for cooling and air conditioning and thereby counter the growing burden on power grids in sunny countries. Less clear, however, is the situation for central European climates. Less than 5 per cent of the total electricity ­produced is used in Germany for air conditioning buildings. However, there is a significantly greater cooling requirement as part of food production and storage, as well as in industrial refrigeration. In these areas the cooling requirement is not temperature- and irradiationdependent to the same extent. Nevertheless, a growing demand for comfort air conditioning can also be expected in Germany, even if the number of full load hours per year will be low in many applications. Solar cooling and air conditioning refers to a process in which solar energy directly supplies a cooling or air ­conditioning process with energy. There is therefore a direct correlation between solar energy and the cooling process. There are three basic approaches for providing cooling with solar energy: • T he photovoltaic generation of electricity and its subsequent use in compression chillers • T hermo-mechanical systems (Vuilleumier and Rankine processes) • Solar thermal systems (desiccant and evaporative cooling (DEC)), ab- and adsorption cooling processes, steam jet cooling process).

This Themeninfo brochure focuses on solar thermal systems. Here it is differentiated between closed and open methods. Closed methods use ab- or adsorption chillers to provide chilled water that is used, for example, in chilled ceilings. Open sorption methods, on the other hand, condition the supply air. Here they not only reduce the temperature but also ensure a pleasant indoor air humidity. In both cases, the system technology and collector system must be matched in terms of the size, suitability and control of the components. Ideally, the solar heat is used for other tasks, such as for domestic hot water or auxiliary space heating. Solar thermally driven cooling and air conditioning technologies have the following ­advantages: • T hey relieve the power grid, since they use little electricity. • The refrigerant (e.g. water) does not have global warming potential. • T hey are mostly operated at temperatures below 100 °C and are therefore suitable for stationary collector technology. • T hey can be combined with waste heat recovery. • T hey do not produce noise and vibrations.

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Closed and open methods Heat assisted systems for air conditioning can be differentiated by the type of method used. Closed chillers provide chilled water that is used, for example, in chilled ceilings. Open sorption methods are used for direct air conditioning, i.e. temperature reduction and dehumidification.

Closed chillers provide chilled water. The chilled water temperature depends on whether devices are supplied that are also used for dehumidification (latent loads) or if the connected room-side components are only used for removing sensitive loads, i.e. to control the temperature. In central air handling units or decentralised circulating air units that are used for controlling both the temperature and humidity of the indoor air, the air is cooled below the dew point. As a result, part of the water vapour from the air is condensed and the absolute humidity decreases. To achieve sufficient dehumidification requires chilled water temperatures in the 6–9 °C range. If, however, the chiller is only going to be used for removing sensitive loads, considerably higher chilled water temperatures in the 15–20 °C range are sufficient. Examples of room-side components include surface cooling systems, i.e. chilled ceilings, underfloor cooling, wall panels with integrated capillary tube mats as well as component cooling or concrete core cooling. Also suitable are other systems for providing silent cooling such as circulating air coolers that work with natural air circulation.

Fig. 1 Principle of an absorption chiller Source: Fraunhofer ISE Pressure [mbar]

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Absorption chillers Absorption chillers are the most widely used technology for thermally driven cooling. They can utilise heat sources at a low temperature level, for example solar heating, ­district heating, industrial waste heat or waste heat from cogeneration plants. Like compression chillers, they ­utilise the dependence of the refrigerant’s boiling point on the pressure. However, the refrigerant is compressed in a dissolved, liquid form in a sorbent. This uses less electricity for the cooling. The most commonly used refrigerant/sorbent pairs are H2O/LiBr and NH3/H2O. ­ H2O/LiBr is usually used for applications above about 4 °C (air conditioning buildings), as this achieves greater efficiency. The advantage of NH3/H2O systems, on the other hand, is the lower freezing point of NH3, which means that useful temperatures considerably below 0 °C can be achieved. The following section, by way of example, examines the H2O/LiBr working pair. The evaporator (E) is at a low pressure level of about 10 mbar. The water refrigerant therefore already evaporates between 4 and 7 °C, and generates the usable cooling capacity by absorbing the necessary evaporation energy. The refrigerant vapour is absorbed by the concentrated LiBr solution in the absorber (A) and, because it is once again in the liquid state, it can be pumped with little energy to a higher pressure level with a solvent pump (SP). By supplying driving heat with a temperature of about 60–95 °C, the refrigerant vapour in the generator (G) is expelled again from the H2O/LiBr solution and is liquefied in the condenser by added cooling water supplied at a temperature of about 30 °C. After being throttled down to the low pressure level, the refrigerant can now be evaporated again in the evaporator. The concentrated solution created in the generator is fed back via a solution heat exchanger (HX) into the absorber where it can once again absorb the refrigerant. Cooling the concentrated solution and preheating the diluted solution in the solution heat exchanger significantly improve the efficiency of the system.

BINE-Themeninfo III/2016

Absorption

Adsorption Condenser

Generator

Driving heat

Generator

Continuous transport of the solution

Evaporator

Refrigerant

Heat rejection

Refrigerant

Condenser

Absorber

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Periodic function reversal

Heat rejection Heat extracted from 'useful cooling'

Adsorber

Evaporator

Fig. 2 Schematic comparison between the absorption refrigeration principle (left) and the adsorption refrigeration principle (right). Whereas in the absorption process steady state conditions can be achieved in the hydraulic circuits by continuous circulation of the liquid solution, cyclic temperature fluctuations occur in the adsorption process. The nominal values for the outputs and temperatures are defined here as mean values over several operating cycles. Source: Fraunhofer ISE

Adsorption chillers

Open, desiccant and evaporative methods

In adsorption chillers, the refrigerant vapour generated in the evaporator is attached to adsorbents. The refrigerant vapour flow and thus the cooling is maintained until the adsorbent is saturated. This then requires regeneration of the adsorbent in order to start the cooling process again. Adsorption chillers therefore operate cyclically. During the regeneration phase, driving heat drives the refrigerant vapour from the adsorbent. The vapour condenses in the condenser. A refrigerant circuit is required to cool the condenser and dissipate the adsorption heat. Compared with absorption chillers, adsorption chillers have a slightly lower thermal efficiency. However, they have the advantage that no pumps are needed in the vacuum range. In addition, there is no danger of crystallisation as in absorption chillers, so that there are fewer constraints on the cooling water temperature.

Open methods are based on a combination of sorptive air dehumidification with evaporative cooling. They make it possible to condition the supply air through an air handl­ ing unit. Not only the air temperature but also the air humidity can be adjusted to a comfortable range. This method, which is known in German-speaking countries as “sorption-assisted conditioning” (sorptionsgestützte Klimatisierung (SGK)), is generally otherwise known as

Steam jet cooling Steam jet cooling technology uses water as the refrigerant and propellant. Heat supplied at high pressure generates motive steam. This vapour is fed through a nozzle and expanded. The accelerated steam generates a negative pressure in the nozzle, whereby water vapour is drawn off by an evaporator. In the evaporator, water can therefore evaporate at low pressure and absorb heat. The depressurised motive steam, which is mixed with the refrigerant vapour from the evaporator, condenses at a medium temperature level and rejection heat is dissipated. The efficiency is significantly influenced by the condensation temperature. In normal operating conditions the thermal EER is less than 1. Steam jet cooling technology is currently only used for a few industrial applications in a very high output range. In research projects, however, attempts are being made to scale the technology down to a small output range and to couple it with solar thermal drive units. Refrigerants other than water are also being investigated.

Efficiency of the cooling output The efficiency of the cooling output is measured by the EER value (Energy Efficiency Ratio). This is the ratio of the cooling capacity to the power input. Air conditioning applications with compression chillers typically achieve EER values between 3 and 4. In addition to thermal energy, thermal chillers also consume electricity, e.g. for control systems, solution circulators, etc. Therefore, two EER values are frequently used. Typical thermal EER values for adsorption chillers range between 0.5 and 0.6; for single-effect absorption chillers they range between about 0.6 and 0.8. Double-effect systems even achieve EER values of up to 1.3. However, they need driving heat at a higher temperature level of about 140–160 °C. The electrical EER of thermal chillers can exceed 50, for adsorption chillers it can exceed 100. However, this figure refers only to the chiller. The power required for the water circuits and the heat rejection is significantly larger than for just the chiller itself, so that the entire system drops to a significantly lower electrical EER value. It is very difficult to provide a precise figure because they are system-specific. A value greater than 8 is also achievable and desirable for small power ratings to ensure that significant electrical energy savings can be achieved relative to compression refrigeration systems.

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Fig. 3 Since 2001 in operation: Solar-powered, open, desiccant and evaporative cooling (DEC) of seminar rooms at the Southern Upper Rhine IHK, Freiburg. The dehumidification rotor is directly regenerated via a 100-m² air collector array, which was installed on the building roof at low cost. Source: Fraunhofer ISE

desiccant and evaporative cooling (DEC). The refrigerant is water and is in direct contact with the atmosphere, which is why it is described as an “open system”. In systems of this type, the supply air temperature and humidity are set according to the comfort requirements, and the necessary fresh air is also supplied simultaneously. Thermal driving energy is required to regenerate the sorbent (expulsion of the water bonded in the sorbent), so that the dehumidification can be maintained. Desiccant evaporative cooling always uses the cooling potential of the exhaust air from the air conditioned rooms, which is relatively cool compared with the ambient air. It therefore requires a closed supply and return air system. The comprehensive air conditioning tasks that are handled using this technique make the direct comparison with water chillers difficult. Common to all technical implementations is a heat recovery unit, which is necessary for the efficient operation of the plant. The most common method is solid sorption using rotating sorption components (sorption wheels). The operating principle is shown in Fig. 4. The sorbent material, such as silica gel or lithium chloride, is embedded in the solid matrix of the sorption wheels. The systems consist of commercially available components; the technical challenge lies in selecting and sizing the components and establishing an appropriate control strategy for the entire system. Analogous to closed refrigeration, in the DEC method the thermal coefficient of performance can be depicted as the thermal Energy Efficiency Ratio (cooling capacity/driving heat capacity), whereby this quotient is then defined only for the periods where the driving heat capacity > 0 (with active regeneration of the sorption components). The cooling capacity is calculated from the enthalpy difference between the ambient air and supply air. An alternative to DEC systems with rotating sorption components is the application of liquid sorption. Such systems dehumidify the supply air using a liquid sorbent (e.g. lithium chloride), which is trickled into the absorber. The sorbent then absorbs water from the supply air. The sorbent is circulated in a cycle; through the introduction of heat, for example from a collector array, water vapour is

expelled and the sorbent is once again ready to dehumidify air. An advantage of this method is that the waterenriched sorbent and the desorbed sorbent can be temporarily stored separately. This enables the air conditioning to be operated outside the operating hours of the collector array. This was utilised, for example, in the construction of the Energy Efficiency Centre in Würzburg (project DEENIFDEENIF, FKZ 0327879A) or in Project Sara, FKZ 0329662D (Storage and conversion of industrial waste heat for air conditioning through open absorption). Until now there have only been a few suppliers of complete systems for DEC systems with liquid sorption. Another, new method uses a supply air-side, sorptively coated crossflow air-to-air heat exchanger. Here the supply air is dehumidified through contact with the sorbent during its passage through the heat exchanger. The sorption heat thereby released is transferred to the exhaust air side of the heat exchanger, is absorbed by the air flowing through and is then delivered by the exhaust air to the environment. Through evaporative cooling on the exhaust air side, the effect is amplified so much that the supply air is cooled and its temperature is lower than the ambient air temperature. This method simultaneously cools the sorption process and increases the dehumidification ­capacity. The process is operated cyclically to periodically regenerate the sorbent. The method has been tested in pilot plants (example: ECOS, FKZ 0327406 A) and is aimed at air conditioning segments with smaller airflow rates < 1,000 m³/h. In DEC systems, the additional components relative to conventional ventilation technology create a higher pressure loss in the air channel, which increases the electrical power requirements for the fans. For a realistic comparison with conventional technology, for example when assessing the primary energy savings, the entire auxiliary energy consumption therefore needs to be considered.

BINE-Themeninfo III/2016

Regeneration heat

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Exhaust air

7 Return air

Ambient air

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HR Heating

Fig. 4 Diagram of a desiccant and evaporative cooling (DEC) system with a sorption rotor and rotating heat recovery (HR) component. The regeneration heat (driving heat) can be provided by a solar collector, whereby a temperature level between 60 and 75 °C is sufficient. Standard cycle with evaporative cooling in the supply air and indirect evaporative cooling in the return air tract. Source: Fraunhofer ISE 1 → 2  S  orptive dehumidification of the supply air; the process is exothermic and the air is heated by the adsorption heat freed in the matrix and by the residual heat from the exhaust air tract. 2 → 3 Pre-cooling of the supply air in the counterflow to the building return air in the heat recovery rotor 3 → 4 Direct evaporative cooling of the supply air with a simultaneous increase of the supply air humidity 4 → 5 Heating register for heating the supply air in winter 5 → 6 Low temperature rise due to the fan 6 → 7 Increase of the temperature and humidity of the air supply through internal loads in the building 7 → 8 Cooling of the building‘s return air by direct evaporative cooling, preferably near to saturation 8 → 9 Preheating of the return air in the counterflow to the supply air in the heat recovery rotor 9 → 10 Supply of regeneration heat to the return air, e.g. from a solar thermal system 10 → 11 Desorption (stripping) of the water bound in the pores of the sorbent material by the hot return air 11 → 12 With the fan, the return air is released to the environment (now exhaust air)

Temperature [°C]

The process flow in a DEC system is depicted in the form of a temperature-humidity diagram in Fig. 5. 0 % relative humidity

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Fig. 5 Temperature humidity diagram showing the process course in a DEC system for the following values: Ambient air (AA) 32 °C, 40 % rel. humidity; Supply air (SA) 20 °C, 60 % rel. humidity. Source: Fraunhofer ISE

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Applications and system selection Depending on the cooling and air conditioning task, different solar thermal assisted systems can be used in the building design. A simplified decision tree helps designers choose between using a chilled water system, desiccant ventilation or a combination of both systems.

Ab- and adsorption chillers can provide low chilled water temperatures for air dehumidification or even work at higher temperatures to provide sensitive cooling, e.g. via chilled ceilings. Absorption refrigeration technology can also be used for process cooling in the temperature range < 0 °C. Several examples demonstrate the high bandwidth of the applications:

Fig. 6 Desiccant and evaporative cooling with solar thermal regeneration of the dehumidification unit. The example shows a standard rotor process for moderate climates. A conventional indoor refrigeration system handles the remaining sensible cooling loads; this can be operated with a high evaporator temperature and thus with high efficiency. Source: Fraunhofer ISE

Collector Storage system Electric compression chiller

Regeneration heat Humidification Exhaust air

> 15 °C Return air Loads

Ambient air Dehumidification (summer)

Chilled water provision from 6 °C for operating circulating air, surface or fresh air cooling: • Only solar thermal cooling provision (without cooling backup system) for increasing the indoor air comfort. Can be used for the residential sector and commercial applications in the low output range. Stationary collector technology is used. The solar thermal system also supports the domestic hot water supply and space heating if required. • Fuel-saver mode, i.e. the conventional (electrically operated) part of the chilled water supply is partially or completely shut down when there is a sufficient solar heat supply. A good correlation between the cooling load and solar thermal output reduces not just the electric power demand but also the peak load. This mode of operation is interesting for both large-scale individual applications as well in cooling networks. In the large output range and in sunny locations, the use of tracking collector technology and multi-effect, thermally driven cooling technology is also possible. • Combination with existing waste heat (from production or cogeneration), taking into account that the existing waste heat utilisation is not displaced.

Refrigerant < 0 °C:

• Used in industrial process cooling, usually in fuel-saver mode. Commercially available are absorption chillers with ammonia water as the working medium. Driving temperatures > 100 °C are generally required that need at the very least stationary evacuated tube collectors or even tracking, concentrating collector technology. • This technology can also be used for cooling buildings, for example in combination with phase change storage systems (e.g. ice storage systems), whereby it may be possible to dispense with backup systems.

Supply air Heat recovery

Heating (winter)

Suitable collectors can generally be differentiated between stationary collectors and tracking collectors with a high concentration ratio. The stationary collectors used include covered flat-plate collectors with selective coatings and various types of evacuated tube collectors.

BINE-Themeninfo III/2016

En passant

In designing the collector array, it needs to be taken into account that there is a small temperature differential in the chiller’s drive circuit (usually about 10 K between the supply and return) and that there are high mass flows. This has implications for the collector connections and the collector control system. Stationary collector technology is used in single-effect, thermally driven refrigeration with driving temperatures < 100 °C. The method is therefore also suitable for regions in central Europe. In most cases, conventional wet heat rejection is still deployed using open or closed cooling towers. However, there is an increasing focus on potential applications with dry heat rejection. Regions with high solar irradiation open up the possibility for using multi-effect absorption refrigeration technology, which requires driving temperatures well in excess of 100 °C and is therefore dependent on tracking, concentrating collector technology. In several pilot and demonstration projects, linear, concentrating collectors have therefore been used for this purpose for the middle temperature range (up to 250 °C). These are parabolic trough or Fresnel collectors.

The temperature levels decide When initially selecting the refrigeration and collector technology in any given application, important aspects include the relationship between the three respective temperature levels for the driving temperature (Thigh ) and the thermal efficiency of the refrigeration as well as the dependence on the temperature lift (Tmean – Tlow ), whereby the temperature lift corresponds to the temperature ­difference between the rejected heat and the chilled ­water. Fig. 9 depicts this relationship. Their high thermal coefficient of performance makes double-­effect absorption chillers particularly interesting because the required heat input for the drive and the required thermal heat rejection capacity are reduced. This leads to smaller collector arrays and lower investment costs for the heat rejection. This contrasts with the increasing driving temperature, which requires the use of concentrating collector technology and thus limits applications to sunny locations. In addition, the use of dry heat rejection is hardly possible, as this leads to an increased temperature lift. However, with a high thermal coefficient of performance, a high temperature lift causes driving temperatures that lie outside the working range of medium temperature collectors and outside the specifications of absorption chillers. In some countries in the southern Mediterranean, the use of desalinated water for the heat rejection in refrigeration systems is problematic or even prohibited; dry heat rejection is therefore usually used. Absorption refrigeration with the NH3/H2O working pair is therefore interesting for these applications with increased temperature lift. This technique offers the advantage that it can also be used in process cooling with temperatures < 0 °C. Until recently, the market availability of thermally driven refrigeration technology was still limited to nominal cool-

Fig. 7 Solar furnace from Mouchot Source: London Permaculture CC BY-NC-SA 2.0 via Flickr

Initial attempts with solar cooling As a result of the shortfall in fuel supplies caused by energyintensive industrialisation, considerable interest in using solar heat developed in France during the second half of the 19th century. Unfortunately, this interest quickly dissipated once new coal deposits were developed and transportation problems for procuring fuel were solved through new railway connections. Nevertheless, solar technology flourished during this time. The beginning of solar cooling can perhaps be dated to 1878. This was the year in which Augustin Mouchot created an ice block with solar heat for the first time, which he carried out on 29 September at the Universal Exhibition in Paris. He used what was then the largest solar mirror device of its time with a roughly 20 m² aperture and a 2-metre-long boiler for directly generating steam as an absorber. The cooling energy was produced using a periodically operating absorption refrigeration process in a Carré Apparatus. This was named after the brothers Edmond and Ferdinand Carré. They developed and operated absorption processes with different working pairs (sulphuric acid/water, ammonia/water). Mouchot reported on his experiment at the World Fair: „On 29 September the sky cleared up around 1 1:30 am, and by noon I had brought 75 litres of water to boiling point. Despite some temporary clouds, the steam pressure gradually rose within two hours to 7 atmospheres; this was the maximum shown by the pressure gauge. I managed to repeat the experiment from 22 September and direct the steam into a Carré Apparatus, which enabled me to receive the first block of ice ever to be produced by the sun.“

Fig. 8 Generation of an ice block on 29 September 1878 during the Universal Exhibition in Paris with a concentrating mirror system and the use of an absorption refrigeration process in a Carré Apparatus. Source: Public domain via Wikimedia

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Fig. 9 The type of solar cooling application and the heat Examples

Surface cooling

Heat rejection

Circulating air cooling

wet/dry

wet

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200

Process cooling < 0 °C

dry

dry

LFC

LFC AbNH3

Ab

D-effect

VRK 100

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FC

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ETC

FPC = flat-plate collector ETC = evacuated tube collector, LFC = linear focussing collector

Ad, Ab Ad, Ab Ad*, Ab* 10

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50

Temperature lift Tmean – Tlow [° C] EERTDC= 1.2

EERTDC= 0.7

Ad *, Ab * = ad- or absorption technology, which under certain conditions can be operated with dry heat rejection

- medium Distribution

Building Start: Cooling load calculation (building parameters: materials, wall structure, geometry, orientation, internal loads, meteorological conditions) ▸ cooling/heating load, hygienic air change

rejection technology used have an effect on both the required driving temperature as well as the choice of collector and thermally driven refrigeration technology. By way of example, two curves are shown for the driving temperature for thermally driven chillers (TDCs) with a thermal coefficient of performance (= EER) of 0.7 (single-effect technology) and 1.2 (double-effect absorption technology). The driving temperatures and working ranges of the collectors and refrigeration technology should only be viewed as indicative figures that can vary depending on the specific product and location (due to the temperature level of the heat rejection). Source: Fraunhofer ISE

Technology

All chilled water system

Climate

Climate

temperate and extreme Installation of centralised air handling unit feasible and desired?

Supply air system, TDC, chilled water network 6–9 °C

TDC, chilled water network 6–9 °C

no Supply air system + chilled water system

yes Building construction appropriate for supply/ exhaust air system (building tight enough)?

temperate and extreme

Climate

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temperate

Climate extreme

extreme

temperate

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Supply/exhaust system + chilled water system

yes All air system (supply/exhaust air system)

DEC, chilled water network 12–15 °C

Conv. supply/ exhaust air system, TDC, chilled water network 6–9 °C

DEC, special configuration for wet climates, chilled water 12–15 °C

DEC

Conv. supply/ exhaust air system, TDC, chilled water network 6–9 °C

DEC, specific configuration for wet climates

Fig. 10 Decision tree for determining methods for providing solar thermal assisted air conditioning in buildings. Based on building services aspects, the distribution medium is established and then the basic technology is selected. The technology marked with the coloured dot could be implemented as in Fig. 8 with a wheel dehumidifier, but also with methods using liquid sorption. Source: Fraunhofer ISE TDC = thermally driven chiller (chilled water); DEC = desiccant and evaporative cooling

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Points of view

ing capacities above about 8 kW. Meanwhile, providers with devices from about 2.5 kW cooling capacity have now entered the market, which means that decentralised solar cooling for the low output range is now within reach. There are few limits to the upward capacity; standard absorption refrigeration technology with H2O/LiBr as the working pair is also available in the MW cooling range. The currently largest solar cooling system is located on the campus of the United World College in Singapore. The system consists of an absorption chiller with a 1.5-MW nominal cooling capacity and a 3,870 m² flat-plate collector array.

Mixed systems for warm locations Open, desiccant evaporative cooling is mainly suitable for supply air dehumidification (treatment of latent cooling loads) and also supports cooling in buildings to a limited extent. However, especially at warm sites the sensible cooling loads are often so large that they cannot be handl­ed by the solar thermally driven air conditioning. Here a separation of the air conditioning tasks makes sense: a solar thermally driven sorptive part ensures the dehumidification of the supply air, while conventional compression refrigeration cools the supply air. The advantage of this method is that the compression refrigerat­ion equipment can be operated at a high evaporator temperature level, as it is no longer necessary to undercut the dew point. It therefore works more efficiently and the power can be reduced. In addition, the subsequent re-heating of the supply air that is often required when undercutting the dew point is eliminated. Fig. 10 shows an example of a possible system configuration with this mode of operation.

Selection criteria A simplified scheme for pre-selecting the basic technology for solar thermal assisted building air conditioning was created a few years ago in SHC Task 25, “Solar Assisted Air Conditioning of Buildings”, as part of the International Energy Agency’s (IEA) Solar Heating and Cooling Programme. The scheme is shown in Fig. 10 and presupposes that the required hygienic air change and cooling loads are already known. Not considered in this simplified approach is the need for backup systems, economic aspects as well as the more detailed selection of tech­ nology (absorption or adsorption, collector type, etc.). The scheme provides guidance in choosing between a purely chilled water system, a purely desiccant and evaporation cooling (DEC) system or a mixed system.

What are the chances for solar cooling?

Prof. Dr Hans-Martin Henning

Head of Thermal Systems and Buildings at Fraunhofer ISE and Deputy Director of the institute. Professor for Technical Energy Systems at the Faculty of Mechanical Engineering at the Karlsruhe Institute of Technology (KIT)

The practicality of solar cooling and air conditioning systems has now been successfully demonstrated in practical operation when there has been a high quality in terms of the design, construction and operational management. Solar cooling is still a complex technology that requires considerable communication between the designers and the installation companies on the heating and cooling side; an interaction that requires greater standardisation of systems on a broad level. With regard to the competing options using renewable energy by means of photovoltaic systems, advanced concepts are necessary in order to underline the advantages of solar thermal cooling in economic terms: such as by additionally using the collector heat for other process purposes and through the efficiency gains in the heat rejection. Highly promising are developments for thermally driven heat pumps in cooling and heating operation. A general advantage of the solar thermal method compared with the solution with PV is that there are no high loads on the power grid. In addition to the considerable potential for avoiding harmful emissions, this provides another argument for trying to increase the market penetration.

Dr Uli Jakob

Managing Director of the Green Chiller Association for Sorption Cooling, Berlin Director of dr. jacob energy research (JER) and Managing Director of Solem Consulting in Europe. Lecturer in the KlimaEngineering degree programme at Stuttgart University of Applied Sciences.

During the last ten years, it has been largely research institutions and innovative companies in Germany which have developed and marketed solar cooling from initial prototypes to marketable products. This means that we now have a diverse range of absorption and adsorption chillers with small and medium cooling capacities available on the market. In Germany, solar cooling is promoted through various subsidy programmes for the private, commercial and trade sectors. Recent estimates indicate that there are more than 1,200 installed solar cooling systems worldwide (end of 2014). This technology is therefore still a niche product but with huge potential. In its Solar Heating and Cooling Technology Roadmap from 2012, the International Energy Agency describes the potential for solar cooling as follows: it is intended that approximately 17 % of the total global demand for cooling shall be covered by 2050 by solar cooling, which corresponds to around 417 TWh/p.a. The markets for solar cooling are seen mainly in Asia and the Middle East. Solar cooling is thus an export product for German companies.

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Planning, costs and integration Correctly designing a complete system for solar cooling is a challenging planning task.

Originalgröße bitte bessere Vorlage

Until now there have only been a few experts who have experience with this technology. This and the currently still high cost of individual components are still barriers for the further dissemination.

The detailed design of the hydraulic components and the control system has a significant impact on the energy efficiency. The separate solar thermal, heating and cooling trades need to be brought together with corresponding care, whereby still incomplete standardisation makes it difficult to compare the performance and efficiency of components (see Standardisation Infobox). Optimisations at several levels reduce the planning and operating requirements. This requires: • Simple design tools (tables, convenient software) • Quality assurance in installation, commissioning and maintenance (recommendations and guidelines) • Standardised assessment methodologies at the component and system level (criteria, guidelines, table and simulation-based methods) • Permanent operational monitoring (e.g. with fault diagnosis)

Fig. 11 Simplified system schematic for the SolCoolSys installations. The heat pump mode is not activated in all plants in the field trial. Source: Fraunhofer ISE

Gas boiler (space heating, DHW) Flat-plate collectors

Heating/Cooling

Storage system

Heat sink/source for the heat rejection, free cooling, heat pump operation

Hydraulic switching Adsorption chiller 8 kW Heat rejection unit

These issues were addressed by scientists and companies in Task 48, Quality Assurance & Support Measures for Solar Cooling Systems, which forms part of the Solar Heating and Cooling Programme run by the International Energy Agency (IEA), who have derived recommendations and framework documents for quality assurance. The Task was completed in 2015 and the documents are now available from the website. Particularly interesting for end users are energy contracting models similar to those already used for solar thermal heating. For solar cooling, the economic hurdles are still high. Nevertheless, such a model has already been implemented by the Austrian solar company S.O.L.I.D. for a large solar heating and cooling system at the United World College in Singapore. The still very small market volume strongly impacts the component prices and the lack of standardisation increases the planning outlay. Both increase the investment costs. With absorption or adsorption chillers with small cooling capacities, large differences in the specific component costs are also noticeable between manufacturers and technologies. Fig. 13 shows an example. The prices of the principal components show that there are considerable economies of scale: in 2011, solar cooling kits (collector, cooling system, peripherals, without installation and cooling distribution) cost around 4,500 euros per kW for systems with a nominal cooling capacity between 8 and 15 kW. With 100-kW systems, the specific costs, however, were around 2,000–2,500 euros/kW. In any event, the investment costs exceed those for conventional cooling several times over. However, it needs to be considered that solar thermal systems can cover additional heating requirements. It is therefore more fruitful to make a comparison based on comparative calculations, as shown in Section 3. There is also potential to significantly reduce the plant and installation expenditure in regards to not just the component costs but also the system development: in the SolCoolSys project, researchers optimised low-capacity systems and tested them in field trials. A total of six adsorption chillers were installed with a cooling capacity

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Fig. 12 Solar cooling as part of the SolCoolSys field tests at the Richard-Fehrenbach vocational school in Freiburg. Left: Adsorption chiller and hydraulic group; right: Collector array. Source: Fraunhofer ISE

between 8 and 15 kW in combination with a flat-plate collector system. A specially designed, pre-fabricated ­ switching group contains all the main hydraulic elements and in some systems also allows the adsorption chiller to be operated as a heat pump and to provide free cooling

via the air cooler with high-efficiency fan technology (Fig. 11). The last system commenced operation in summer 2013 at a vocational school in Freiburg (Fig. 12). There existing borehole heat exchangers are also used as heat sinks for the heat rejection.

Fig. 13 Price searches carried out for thermally driven chillers between 2010 and 2012, converted to the specific

Euros/kW

costs per kW nominal cooling capacity. Prices exclude VAT, without heat rejection units and other peripherals. The solid lines were used as cost curves in the Solar Cooling in Buildings comparative study as part of the joint EVASOLK project. Source: Fraunhofer ISE 2,000 1,800 1,600 1,400 1,200 1,000 800 600 400 200 0 1 Nominal cooling capacity [kW]

10

100

Search 1; AbTDC T > 0 °C

Search 1; AdTDC

Search 1; AbTDC T < 0 °C

Search 2; T > 0 °C

Search 2; T < 0 °C

EVASOLK; S-effect, T > 0 °C

Search 3; AbTDC T > 0 °C

Search 3; D-effect

EVASOLK; D-effect

1,000

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Research starting points Solar cooling became an integral part of energy research at the latest with the launch of the Solarthermie 2000plus funding concept. Subsequently, different solar power systems were successfully further developed as an integral component of the building services technology within research and demonstration projects. A current focus is on the development of components.

In Solarthermie 2000plus, the operation of five systems with very different operating concepts were analysed. As part of the accompanying research (FKZ 0329605A), the researchers demonstrated that additional collector systems can also be successfully integrated into largescale existing plants with a thermally driven cooling supply. An example is provided by the solar cooling of operational buildings belonging to Deutsche Telekom at its site in Rottweil. In Rottweil a solar thermal system reduces the use of gas boilers. Most of the heat used to drive the absorption chiller is also provided here by waste heat from a CHP plant. In contrast to the system at FESTO (see “In practice: Success Stories”), the heat supply systems are not operated here simultaneously but sequentially. Both concepts make different demands on the collector size, storage size and operating strategy. The diagram in Fig. 14 shows the main components of the system in Rottweil. Two other installations demonstrate autonomous solar cooling in the low output range (without cooling backup; gas boiler used only for space heating and/or domestic hot water supply purposes). In the low-energy building belonging to the Vocational and Technical School in Butzbach, two absorption chillers with 10-kW nominal cooling capacities air condition the classrooms. They provide chilled water at two different temperature levels: one for supply air cooling and dehumidification, and the other for operating the surface cooling system. The system is also used for training purposes in the technical field. A schematic of the system is shown in Fig. 15. In Fürth, an absorption chiller with a 30-kW nominal cooling capacity has cooled the office building of the IBA AG since 2007. The heat is provided by a flat-plate collector with an 88 m² aperture area. Both in Butzbach and Fürth it was shown that small plants can be used to provide solar cooling for several hours a day (up to 8 hours in Butzbach) and that the indoor climate is effectively enhanced in the buildings without the need for additional backup for the cooling provision.

In terms of the technology, the plant operation for the solar cooling in Solarthermie 2000plus ran largely trouble-­ free. Whilst the operation of the collector arrays was positively evaluated, the researchers were not always satisfied with the electrical performance factor for the solar cooling. They see potential for optimisation mainly in the design and operation strategy for the heat rejection, but careful sizing of the circulation pumps in all hydraulic circuits is also essential.

Heat rejection with phase change storage systems The efficiency of thermal chillers is highly dependent on the heat rejection temperature provided. Dry cooling towers are pushed to their limits on very hot days when there are extremely high cooling requirements. In the SolarCool+PCM project, researchers from ZAE Bayern therefore developed a concept in which a phase change material storage system (PCM storage) absorbs such load peaks. The latent heat storage unit receives a portion of the waste heat at a constant temperature of 29 °C through melting the salt hydrate, calcium chloride hexahydrate. During the following night, the storage system discharges again through the heat rejection unit. A demonstration plant was developed, installed and continuously optimised in 2007. The PCM storage system is integrated on the heat rejection side of a 10-kW absorption chiller, which is used via chilled ceilings for cooling offices. This therefore enables a sufficiently low condensing temperature of 32 °C to be ensured even at high ambient temperatures. After the optimisation, the electrical EER for this system achieved an average value of 11, which is considerably more than comparable compression refrigeration systems.

Heat rejection in existing refrigeration systems The heat rejection in solar thermal refrigeration systems greatly affects the performance and efficiency of the sorption chiller unit and also often requires significant power.

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Evacuated tube collector 315 m² aperture

Absorption chiller 2 x 340 kW

Storage tank 20 m³

Storage tank 5 m³ Cooling

Storage tank 25 m³

Emergency cooler CHP 315 kWth

Space heating, DHW Heat rejection (wet, open)

Gas boiler

Fig. 14 Solar thermal assisted cooling of operating facilities and offices at Deutsche Telekom‘s operations base in Rottweil. The collector system is installed on a neighbouring building and commenced operation in 2011. Source: Schematic: Fraunhofer ISE; Photo right: Offenburg Secondary School

Condensing boiler 28 kW

Absorption chiller 2 x 10 kW Storage tank 1 m³

Supply air (2 x 1,250 m³/h)

Storage tank 2.9 m³

Evacuated tube collector 60 m² aperture

Chilled ceilings (110 m²) + cooling shaft Heating Heat rejection (wet, open)

Fig. 15 Solar thermal cooling in the low-energy building belonging to Butzbach Vocational School. To test different operating strategies, the drive circuits for both chillers can be connected in series or parallel. The plant commenced operation in 2009. Source: Schematic: Fraunhofer ISE; Photo right: Butzbach Vocational and Technical School

In the SolaRück project (www.solarueck.de) coordinated by Fraunhofer ISE, scientists have analysed the operation of heat rejection units in existing refrigeration systems. They investigated how the heat transfer can be improved and auxiliary energy simultaneously saved. In addition, they developed generic management strategies for heat rejection units and complete systems in order to increase the energy efficiency of the overall system. The industrial partners experimentally tested different concepts for innovative heat rejection systems: • Adiabatic pre-cooling of the air to lower the cooling water temperature at high ambient temperatures • Small-capacity hybrid coolers with minimal maintenance requirements • Dry coolers based on plastic materials for reducing weight and costs

Collector-integrated sorption components In collaboration with Fraunhofer ISE, the Vaillant company has investigated innovative system solutions for integrating solar-generated heating and cooling as part of the KollSorp research project. One concept is based on the cyclic operation of collector-integrated sorption modules. Hygroscopic salts provide a suitable working pair here, which through water absorption enable a heat transformation process. A functional prototype was tested in Fraunhofer ISE’s solar simulator. The technical objectives were achieved. In particular, the system configuration and the LiCl-H2O working pair provide the following advantages:

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Solar collector

Heat exchanger

Buffer storage

Backup

Hot side

45 °C T

E

T

G

T

T

B A

Main cooler

F Auxiliary cooler

Heat rejection B C

PCM A

A

C D

Load

F E Backup

Cold side

D C

E

Fig. 16 Schematic diagram of the SolarCool+PCM cooling concept By integrating a phase change storage tank in the heat rejection system, a low return temperature for the cooling water can also be ensured even with high outside temperatures. This therefore combines the advantage of a wet cooling tower, which has a low heat rejection temperature, with the advantage of a dry cooling tower, which provides ease of maintenance. The storage system is regenerated by releasing heat to the night-time air. This enables the heat rejection to be shifted to the night-time hours, which enables the use of low outside temperatures. Source: ZAE Bayern

• Low system complexity • Desorption temperatures > 100 °C possible without problems • High thermal efficiency (working pair, low loss) • High electrical efficiency with electrical performance factor > 12 (heat rejection is distributed during the day and night, high heat rejection temperatures possible, no pump for high temperature circuit) Other investigated concepts include the coupling of solar thermal systems, backup (gas burner) and sorption modules to efficiently generate heat, hot water and cooling energy.

Plate heat exchangers replace shell and tube heat exchangers In the “Absorption chillers based on compact plate devices” research project, researchers from ZAE Bayern are investigating the extent to which absorption chillers with low outputs (3-5 kW cooling capacity) can be built with plate heat exchangers instead of the usual tube heat exchangers. They hope that this will significantly reduce the volume and costs. This will therefore also enable absorption chillers with small capacities to achieve a better foothold in the mass market. For this purpose, various plate geom-

etries are being tested and a pilot plant based on plate heat exchangers is also being constructed.

Development of a solar thermal, heat pump-based heating and cooling system Since with solar air conditioning a backup system for generating cooling energy must always be maintained to ensure specific comfort or process requirements, this results in an increase in space requirements and equipment costs. ZAE Bayern is therefore researching the integration of this backup system in the form of a direct-firing gas burner in a multi-effect absorption chiller. It is planned to develop a prototype as part of the research project. Since the thermal EER of a single-effect absorption chiller is very low (0.7), it is intended that the heat shall be coupled via the gas burner in a second stage of the absorption chiller. This enables a thermal cooling efficiency of 1.2 to be achieved. Furthermore, the integrated gas firing also enables efficient heat pump operation in winter. The gas burner can assume conventional heating tasks if the heat pump cannot be operated due a lack of ambient heat. The development also aims to furnish the integrated solution with a pre-assembled control and hydraulic unit so that the system always operates in its most efficient mode and the integration of the system in the planning and operation is simplified for end users.

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Day Condensation

Desorption

Evaporator/condenser

Reactor/absorber

Night Evaporation

Absorption

Fig. 17 Operating principle of a collector-integrated sorption module: Sunlight activates (dries) the working medium in the absorber; the condenser has to be cooled. Cooling can be generated at night by evaporation of the water; the reactor/absorber needs to be actively cooled. The right image shows the functional prototype on the solar simulator. Source: Fraunhofer ISE

Fig. 18 A cross-section through a modified plate heat exchanger; outwardly the plate heat exchanger looks similar to a conventional model. The internal structures and fixtures enable not only the transfer of heat but also the transport of substances. The same functionality is therefore achieved as with a shell and tube heat exchanger but in a more compact design and with less weight. Source: ZAE Bayern

Summer Solar cooling – single-effect (SE)

Solar assisted cooling

Double-effect (DE) cooling (fossil)

Gas burner

Gas burner DE

90 °C SE 15 °C

40 °C

COPTDC = 0.7

90 °C SE 15 °C

DE 40 °C

SE 15 °C

COPTDC = 1.0

40 °C

COPTDC = 1.2

Spring / Autumn / Winter Solar direct heating

Heat pump operation

Boiler operation (fossil) Gas burner

Gas burner

35 °C

DE 35 °C

Solar LTheating 10 °C Or: LT-geothermal energy 10 °C

35 °C COPHP = 2.2

Fig. 19 Operating modes for the solar thermal, heat pump-based heating and cooling system. Source: ZAE Bayern

1,200 °C

COPHP = 1

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In practice

Aus der Praxis

Absorption cooling for different regions In many sunny regions, high ambient temperatures combined with a lack of fresh water present a problem when operating solar cooling with wet heat rejection. The use of dry heat rejection, on the other hand, limits the thermally driven technology that can be utilised. Together with commercial partners, Fraunhofer ISE has therefore tested a concept for solar thermal process cooling in the temperature range between 0 °C and –10 °C which is specifically aimed at cold storage in warm, sunny regions. Here an air-cooled absorption chiller with the NH3 /H2O working pair is powered using solar heat from a Fresnel collector array. The pilot structure for the plant was built by project partner Kramer Kühlraumbau GmbH in Umkirch/Freiburg.

The target market for later applications is southern Europe and North Africa.

Fig. 20 Pilot plant for solar cooling storage. The cold storage in a low-rise wing of the building also includes an ice storage system. The Fresnel collector and absorption chiller are installed on the roof. Source: Kramer GmbH

Fig. 21 380 m² of evacuated tube collectors (left)

A typical example of a solar thermal cooling system in Germany is the plant constructed in Altensteig in 2012. The sloping roof of the technical centre for a production facility is used for generating solar heating, cooling and electricity. The solar heat drives a low-temperature water/lithium bromide absorption chiller. The systems, which were initially designed for utilising waste heat from motorised CHP plants and district heating systems, can be operated with comparatively low driving temperatures (e.g. heating water at 86/71 °C with chilled water at 9/15 °C), and are therefore also ideal for using solar thermal heat. The continuous mode of operation and the flexible operation over a wide range of external temperatures are further advantages of the absorption technology.

on a roof belonging to Friedrich Boysen GmbH & Co. KG in Altensteig provide heat to drive a 150-kW water/lithium bromide absorption chiller (right). Source: EAW Energieanlagenbau Westenfeld GmbH

Air-cooled sorption chillers Indirectly heated sorption chillers are required for solar thermal cooling, i.e. systems that can be operated with heating water or heating steam. Until now, indirectly heated sorption chillers have only been available with water-cooled condensers and absorbers/adsorbers, and require an additional heat rejection unit. In comparison, air-cooled systems have the following advantages: • Reduced system complexity • Reduced installation costs • Reduced auxiliary energy demand by eliminating the cooling water circuit. In the “Air-cooled Sorption Chiller” project, the Institute for Air Handling and Refrigeration (ILK Dresden) developed and built a directly air-cooled absorption chiller with the water/LiBr working pair. After it was measured on a test rig, a field trial was carried out in summer 2015 which was able to verify its functionality and output capability.

Solar heating and cooling in northern and central Europe (FKZ 03ET1231A) Since heating is the dominant energy mode in northern and central Europe, ZAE Bayern is developing a system that – without additional auxiliary units – meets not just the cooling demand but also in particular the space heating and domestic hot water requirements. Here an absorption heat pump is combined with a solar thermal collector and a biomass-fired boiler plant. The solar energy is used both as driving energy for the sorption chiller as well as a low-temperature heat source for the absorption heat pump in heating mode. An integrated wood pellet-fired boiler plant is used as backup for the driving energy. This can provide the driving energy for both the heating and cooling modes if there is insufficient solar energy.

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In practice Success stories

Fig. 22 In operation since 1991: Solar cooling of a wine bottle warehouse in Banyuls in southern France. Two absorption chillers with a total nominal cooling capacity of 52 kW cool approximately 3,500 m² of warehouse space. The thermal inertia of the bottle store means that a cold storage facility is not necessary. The plant is driven using an evacuated tube collector array; there is no backup system. Source: Tecsol, FR

One of the solar cooling success stories is a wine warehouse in Banyuls, southern France, which is cooled purely using solar thermal cooling. The plant commenced operation in 1991 and has therefore been working for over 20 years. An absorption chiller with a 52-kW nominal cooling capacity and a 215-m² evacuated tube collector array cool the three floors of the wine bottle warehouse via a ventilation system. About 3 million wine bottles are stored in the warehouse across 3,500 m² of usable space. A one-cubic-metre hot water storage tank is designed to provide only short-term storage. The wine bottles themselves act as the cooling storage system. Another example is the solar assisted cooling of the technology centre belonging to FESTO AG & Co. KG in Esslingen-Berkheim. An adsorption chiller with a 1.05-MW nominal cooling capacity has air conditioned the building complex since 2001. Waste heat from the production facility and heat from gas boilers is used to drive the three adsorption chillers, which each have a 350-kW nominal cooling capacity. As part of the Solarthermie 2000plus funding programme, the plant has been expanded to include a large evacuated tube collector array that largely reduces the use of the gas boiler to drive the cooling supply. The collector area is 1,218 m² in size (aperture area). With a total of 17 m³, the hot water tank here is also designed to provide just temporary storage; the cooling technology‘s high number of operating hours means that solar heat is always absorbed immediately. Only water is used as the collector fluid; a special frost protection switch in the collector control system prevents damage to the collectors during the winter.

Fig. 23 above: In operation since 2001/2007: Solar assisted air conditioning of the technology centre at Festo AG & Co. KG. One of the three adsorption chillers, which each have 350-kW nominal cooling capacity, can be seen on the left. Source: Fraunhofer ISE below: The collector array was added in 2007. Waste heat and solar heating utilisation have priority; heat is additionally used from the gas boilers if there is an increased cooling demand. Source: Offenburg Secondary School

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Solar power instead of heat Thanks to the falling cost of photovoltaic energy, solar-electric cooling with compression chillers is becoming increasingly attractive. In the joint EvaSolK project, researchers analysed the competitive situation of solar thermal systems for different buildings, climate zones and configurations, including in comparison to conventional cooling technologies.

In the EvaSolK project, which stands for “Evaluation of the opportunities and limitations of solar cooling in comparison to reference technologies”, the researchers demonstrated the potential for different applications. Criteria for the evaluation were the primary energy demand, CO2 emissions and the economic efficiency. As reference, the researchers investigated scenarios with conventional compression chillers. In comparing the solar thermal and photovoltaic systems, the scientists examined the overall balance of the building supply at the annual level, i.e. for heating, cooling and domestic hot water heating. For their model calculations, they chose five sites representing the most common climate types in central and southern Europe. Other calculations cover very sunny and warm locations (Antalya, Turkey and Bechar, North Africa). Here concentrating collectors and double-effect absorption chillers can be used. The researchers analysed three applications or uses: A Buildings whose usage structure roughly corresponds to an apartment building with six residential units. In addition to the cooling and heating load, the hot water requirement is also taken into account. B Office buildings where the use concentrates on the workspaces used during workdays. The hot water demand is low. It was differentiated between (B) small buildings with two floors and (B+) larger buildings with eight floors and a ventilation system. C Buildings with an increased use in the evenings, including at weekends. A hotel is used as a model. Here there is also an increased domestic hot water requirement. The calculations differentiate between buildings with two floors (C) and buildings with eight floors and a ventilation system (C+). Specifically, the researchers simulated the following configurations: Versorgung

ST – solar thermal assisted building supply • Single-effect ad/absorption (double-effect at two locations)

• Flat-plate collector, evacuated tube collector (double-effect: concentrating collector) • Backup cooling supply: Water chiller; in appropriate applications also solar thermal autonomous cooling • Heating backup: Gas boiler (only space heating, DHW)

Reference

• Cooling system: Electrically driven compression refrigeration; in accordance with the building type and size: Multi-split units, water chiller, • Heating supply: Gas boiler.

Reference + PV (grid-connected)

• Building supply: As in reference, • Additionally: Grid-connected PV generator; no additional components (storage system), • PV rated power: 50 % of the electrical rated power consumption of the refrigeration. In the simulations, the collector array size was optimised for minimal solar surpluses. The nominal capacities of the PV systems were limited to 50 % of the rated power consumption of the electrical compression refrigeration to achieve a high economic efficiency. More than 70 % of the PV electricity can therefore be absorbed directly by the supply technology and used by other loads. The surplus electricity fed into the grid was considered in primary energy and emission terms.

Economic assessment The evaluation parameters, such as the relative primary energy savings ΔPErel and the costs of the primary energy savings ΔCPE, were calculated for an operating period of 20 years. The evaluation variables are related to the reference. Example: Relative primary energy savings ΔPErel = 0.2 means 20 % less primary energy costs compared with the reference. ΔCPE indicates how the costs of the primary energy saved (kWh) in the solar variant compare with the costs of the reference (> 0: no amortisation within the operating ­period; < 0: the life cycle costs are lower than those of the reference).

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min. ΔCPE [euros/kWhPE]

Application A without cooling backup 0.5 0.4 0.3 0.2 0.1 0 – 0.1 0.0

0.2

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rel. PE savings ST – solar thermal assisted building supply Freiburg

With the office buildings (B), the solar thermal variants are also far from achieving cost neutrality. The potential for ­primary energy savings is considerably less than in application A. Noticeable here are the small domestic hot water requirement and the low load requirement at weekends. In terms of primary energy savings and the specific costs of the PE savings, the Ref+PV option is more advantageous here. Much more favourable is the load profile of a hotel (C). Here the high domestic hot water requirement makes the solar thermal variants more advantageous. The cooling backup makes it possible to design the thermally driven cooling to meet about 1/3 of the cooling peak load (Fig. 25). This significantly improves the economic efficiency. Since the peak power is only rarely required, the primary energy savings reduce only moderately. Here the solar-electric variant is also more economically attractive as a whole. However, the solar thermal systems come close to the reference in southern European locations. In particular the CO2 savings are advantageous. Assuming cost reductions realisable in the medium term of –25 % for the solar collector and –33 % for the cooling system, the efficiency at southern locations is comparable to conventional and photovoltaic systems. Solar thermal systems, however, possess the greater potential for primary energy savings. In summary, the researchers conclude that, owing to the still high costs, solar thermal cooling is the most promising when an additional heat demand leads to a uniformly high utilisation of the collector. Examples include hotels, hospital departments, etc. Further calculations show that

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Fig. 24 System simulations for the application area A. The ST configuration is operated here without a backup system for the cooling supply. The solar coverage of the cooling demand with solar thermally driven refrigeration is always > 70 %. Within the cooling season, this may mean that the target values for the indoor temperature are exceeded to a limited extent. This is tolerable for residential buildings. The Reference+PV configurations have lower costs, but save less primary energy. Source: Fraunhofer ISE Application C +; PTDC 33 % min. ΔCPE [euros/kWhPE]

For small residential buildings (A), solar thermal assisted cooling turns out to be uneconomical with current costs. There is also little potential for cost reductions. This applies in particular to less radiation-rich locations with relatively few cooling operation hours. However, the CO2 savings are high. Whenever possible, a backup system for the cold supply should be dispensed with.

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Fig. 25 System simulations for the application area C+. The thermally driven chiller (TDC) in the ST configuration was designed here not at full capacity but at 33 % of the maximal cooling load. The plant has a compression refrigeration backup. Source: Fraunhofer ISE

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Fig. 26 Solar thermal cooling competes with systems with conventional or solar electric driven compression chillers. Left: Absorption chiller Source: Kramer GmbH; Right: Compression chiller Source: Gerd Hirn, BINE Information Service

in regions with very high solar radiation, highly efficient ­double-effect absorption refrigeration achieves costs ­comparable to the reference. The solar electric variant is, however, slightly more economical. In addition to carefully ­dimensioning the thermally driven cooling, it is therefore also essential that the costs for the main components are reduced.

Fig. 27 Application as shown in Fig. 25, but lowered investment costs in the ST configuration: Collector system –25 %; thermally driven cooling system (incl. heat rejection) –33 %. Source: Fraunhofer ISE

min. ΔCPE [euros/kWhPE]

22

0.15

Better evaluation of environmental effects The evaluation parameter ΔCPE solely measures the costs of the primary energy savings. However, the actual primary energy saved remains unconsidered. As a compromise between purely economic and purely environmental considerations, the researchers therefore defined a dimensionless parameter Opt+. They used this to total the cost and primary energy savings with equal weighting (each standardised to the costs in the reference). The evaluation parameter therefore compensates for the cost disadvantages caused by high primary energy savings. Advantageous compared with the reference are values > 0 for Opt+.

Grid relief

Application C +; PTDC 33 %

Especially in southern climates, the impacts on the often weak electricity grid play an important role. The investigations show that solar thermal methods reduce “grid stress”, whereas PV systems tend to increase it – despite the high proportion of self-consumption. In the solar thermal variant, in particular the configuration without cooling backup (solar thermal autonomous cooling) has a lower maximum. In Ref+PV, no additional storage systems (thermal or electric) were considered. These can reduce the grid fluctuations with appropriate regulation, but require noticeably higher investment costs.

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min. ΔKPE [Euro/kWhPE]

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Standardisation

Application C +; PTDC 33 %

Extensive standards and test specifications exist for conventional components for heating and cooling systems that enable different manufacturers to be compared with one another. In the (solar) thermally driven cooling sector, however, the regulations are incomplete. Even just at the component level there are still no standards for hot water-driven cooling devices that are applicable within Europe.

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Fig. 28 Application C+ : Comparison of ΔCPE and Opt+.

With the optimisation parameter Opt+, not just the configurations from Ref+PV but also the ST configurations lie in the positive range at several locations. The investment costs were not lowered here. Source: Fraunhofer ISE

• T he BIN method, which links part load values of the components with operating frequencies as part of a tabular procedure. This method is currently used, for example, in the standard for assessing the seasonal efficiency of electric heat pumps (DIN EN 14825: 2013). • T he CTSS method (Component Testing System Simulation): Here the key system components are measured individually and parameters set that are then used in simulations to determine the system performance under reference conditions. This is used, for example, to calculate the yield from solar thermal hot water systems (DIN EN 12977: 2012).

Application A, Athens fGrid/fGrid, Reference

For this reason, a uniform evaluation method is currently being developed as part of EA-SHC Task 48 (Quality Assurance and Support Measures for Solar Cooling Systems), whereby two different approaches are being compared.

1.8 1.6

The results can be drawn upon, for example, in subsequent DIN activities. Australia was the first country to publish a „Solar Cooling Standard“ in September 2013. This works according to the CTSS method and so far only includes open methods.

1.4 1.2 1.0

Fig. 30 Solar cooling of the wholesale

0.8

market building in Oberkirch, Germany Source: p-power GmbH

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0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

max|PGrid| / max|PGrid, Reference | Reference

Ref+PV; n = 1

ST

ST; without backup

Ref+PV; n = 0,5

Fig. 29 Grid interaction index fGrid, shown relative to the maximum

value of the electrical power in/from the grid. The values are standardised to the reference values. fGrid represents a measure of the fluctuations in the electricity exchanged to and from the grid (grid input and output): an increasing value corresponds to greater „grid stress“. Two data points are included for the Ref+PV variant: for the standard configuration used in the comparative study for the PV generator, covering 50 % of the electrical power consumed by the compression chiller (n = 0.5), and for covering 100 % of the electrical power consumption (n = 1). Source: Fraunhofer ISE

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BINE-Themeninfo III/2016 I/2015

Überschrift Outlook Solarhinten, Weit coolinghinter and airden conditioning Wortbergen, hasfern been dershown Länder toVokalien be particularly und Konsonantien advantageousleben in die Blindtexte. sunny locationsAbgeschieden with high operating wohnen hours. sieFor in Buchstabhausen German companies, an the derbest Küste opportunities des Semantik, therefore are eines großen provided Sprachozeans. by exportsEin to tropical kleines countries. Bächlein namens Researchers Duden and fließt companies durch ihren Ort undthat expect versorgt the economic sie mit den viability nötigen of the Regelialien. various technologies Es ist ein paradiesmatisches will improve. TheyLand, see in dem einemdevelopment particular gebratene Satzteile potential, in for denexample, Mund fliegen. in increasing Nicht einmal the thermal von der and allmächtigen electrical Interpunktionfactors performance werden of sorption die Blindtexte chillers.beherrscht Decisive, however, – ein geradezu will be further unorthographisches cost reductions Leben. in the components Eines Tagesand abersystem beschloß technology. eine kleine Researchers Zeile Blindtext, and providers ihr Name arewar working Lorem onIpsum, hinausthe simplifying zu system gehen in and diethus weite reducing Grammatik. costs, especially with devices in the low power range. Starting points include, for example, the integration of circulation pumps Der große (driving heat, Oxmox heat riet rejection, ihr davon chilled ab,water) da es as dort well wimmele as integrated von bösen dry heat Kommata, rejection. wilden Fragezeichen und hinterhältigen Semikoli, doch das Blindtextchen ließ sich nicht beirIn the project planning, the economic and ecological benefits can only be achieved ren. Es packte seine sieben Versalien, schob sich sein Initial in den Gürtel und machte if optimum use is made of the collector array. Comparative calculations show that: sich auf den Weg. Als es die ersten Hügel des Kursivgebirges erklommen hatte, warf es einen letzten Blick zurück Skylinehigh: seiner Heimatstadt die • The environmental impactsauf aredie generally depending on theBuchstabhausen, type of application, Headline Alphabetdorf die Subline eigenen Straße, der Zeilengasse. up to 80von % primary energyund savings can be seiner achieved in sunny locations. If, however,Wehmütig lief ihmenergy eine rhetorische über die Wange, dann setzte es seinentoWeg fort. only electric is saved by Frage the renewable supply, it is currently difficult achieve economically beneficial operation. Die Copy warnte das Blindtextchen, da, wo sie herkäme wäre sie zigmal umgeschrieben • The economic efficiency improves if the solar thermal system is additionally used worden und alles, was von ihrem Ursprung noch übrig wäre, sei das Wort „und“ und das for supporting space heating and, in particular, domestic hot water heating. The costs Blindtextchen solle umkehren und wieder in sein eigenes, sicheres Land zurückkehren. become closer to conventional supply technology, whereby the life cycle costs of Doch alles Gutzureden konnte es nicht überzeugen und so dauerte es nicht lange, bis solar thermal variants are still generally higher. In particular, applications with very ihm ein paar heimtückische Werbetexter auflauerten, es mit Longe und Parole betrunkhigh additional domestic hot water demands (e.g. hotels, hospitals) are favourable en machten und es dann in ihre Agentur schleppten, wo sie es für ihre Projekte wieder in this respect. und wieder mißbrauchten. • The correct configuration of large systems plays an important role: if cooling backup is available (conventional refrigeration unit), it makes sense not to configure the absorption or adsorption chiller to cover peak loads. Optimised configurations to meet cooling capacities significantly below the peak load make effective cost savings without significantly losing primary energy savings.

Impressum Imprint Project organisation Projektorganisation Bundesministerium Federal Ministry for Economic für Wirtschaft Affairs and und Energie Energy (BMWi) (BMWi) 11019 Berlin Germany Projektträger Jülich Forschungszentrum Project ManagementJülich JülichGmbH 52425 Jülich Forschungszentrum Jülich GmbH 52425 Jülich Förderkennzeichen Germany 00327430M 0327430H Project number 0327387A-D 0325966A,B,C 0335007P 0325979A 0325994 ISSN 0325997 1610-8302 0327406A 0327875A Herausgeber 0327879A FIZ Karlsruhe · Leibniz-Institut 0329662D für Informationsinfrastruktur GmbH 03ET1107A Hermann-von-Helmholtz-Platz 1 03ET1213A 76344 Eggenstein-Leopoldshafen ISSN 1610-8302 Publisher FIZ Karlsruhe · Leibniz Institute for Information Infrastructure GmbH Hermann-von-Helmholtz-Platz 1 76344 Eggenstein-Leopoldshafen Germany

• Solar thermal configurations generally help to reduce fluctuations in the electricity drawn from the grid connection. This should be especially taken into account in regions where there is a high utilisation of the electricity grids through air conditioning.

Links und Literatur

>> www.XXX.de >> www.XXX.de >> www.XXX.de >> www.XXX.de >> www.XXX.de >> Literaturhinweis >> Literaturhinweis >> Literaturhinweis >> Literaturhinweis >> Literaturhinweis >> iteraturhinweis >> LProject reports on EVASOLK, Solarthermie 2000plus, ECOS, AgroKühl >> Lwww.solare-kuehlung.info iteraturhinweis >> >> LQiteraturhinweis uality Assurance and Support Measures for Solar Cooling Systems

Links and literature

http://task48.iea-shc.org/ >> Green Chiller – Association for Sorption Cooling, Berlin www.greenchiller.de/indexeng.php >> H enning, H.-M. (Ed.); Motta, M. (Ed.); Mugnier, D. (Ed.): XXX. BINE-Projektinfo XX/20XX Cooling Handbook. A Guide to Solar Assisted Cooling and >> Solar XXX. BINE-Projektinfo XX/20XX Dehumidification Processes. Vienna (Austria): AMBRA/V, Sprache 2013. 3rd issue, >> D ieses Themeninfo gibt es auch online und in englischer unter ISBN 978-3-99043-438-3 (print issue) www.bine.info/Themeninfo_X_20XX

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