PILOT PLANT FOR HIGH TEMPERATURE TES SYSTEMS Pablo Arce1, Antoni Gil1, Joan Roca1, Marc Medrano1, Servando Alvárez2, Luisa F. Cabeza1 1

GREA Innovació Concurrent Edifici CREA, Universitat de Lleida, Pere de Cabrera s/n, 25001-Lleida (Spain) Phone: +34-973 003576, Fax: +34-973 003575 e-mail: [email protected] 2 AICIA - Universidad de Sevilla Escuela Superior de Ingenieros, Camino de los Descubrimientos s/n, 41092-Sevilla (Spain) ABSTRACT Over the last 25 years Thermal Energy Storage (TES) has been gradually becoming one of the main research topics due to its high possibilities of application in many technological fields. High Temperature Thermal Energy Storage (HTTES) is now a very appealing topic for both scientists and researchers. Among the recognized HTTES technological options are the use of thermoclines, passive storage systems using concrete, latent storage systems, two-tank systems, etc. However, none of them has been designed to be able to work with more than one type of storage material as well as to overcome issues derived from operation like, for example, corrosion. The goal of this work is the design of a pilot plant with a thermal storage capacity of 10-15 kW, which can be capable of using both static and pumpable heat storage materials, also being able to study other aspects that affect the different components of the system. 1. INTRODUCTION Thermal energy storage (TES) can perform a large contribution to energy savings within the industrial field, as well as to make possible an efficiency increase of the diverse stages which are part of each productive process in particular. Currently there are different energy storage methods: two-tank molten salts, thermal oil with two tanks, one tank thermocline, one tank thermocline using filling material, etc. On the other hand, TES industrial applications cover a wide range. Some of them are: electric power storage, ice thermal storage, water thermal storage, solar water-heating, solar air heating, offpeak electricity storage, air conditioning (chilled water systems), waste heat recovery, combustion gas turbine air inlet cooling, food processing, manufacturing of construction materials, production of paper, textile industry, water purification, desalination, double effect sorption cooling, metallurgy, ferrous and non-ferrous metal casting, ceramics manufacturing and glass manufacturing. It may be seen that a common factor to all the previous is the probable or certain presence of not entirely used energetic “inputs” due to each process particularities, “inputs” that may be actually reutilized in the same task or a related one in order to correct the disparity between the required energy and the one really being used. It is based on these ideas that this document primary goal is to introduce the design of a TES pilot plant. This plant or system is capable of simulating the behaviour of an energy supply source in terms of temperature fluctuation and the effect of this fluctuation over the whole TES (Arce 2008).

In more detailed terms, the primary objective of this plant is the charging and discharging of thermal energy on diverse TES systems in order to test them within a 200 – 400 ºC range. The secondary objectives are: • to provide or remove heat from storage when needed and in precise quantities • to demonstrate that the required technology for this is already available The designed system is able to perform: • Either charging or discharging at full/partial system capacity, both continuously or not. • A combination of any of the previous ones 2. PLANT REQUIREMENTS The plant requirements may be classified as follows: Operational: • Maximum storage temperature of 390-400 ºC. • Capability of supplying heat to different types of thermal storage materials and/or systems. • A 4-hour desired heating time (or the closest value) for the heat transfer fluid. • A cooling down sub-system to simulate discharging in a short time, enabling the system to start a new charging process under equal or different conditions. • The use of water, oil or other thermal fluid as cooling media until reaching the custom-set cooling operation temperature. Control: • The system must stop if: o There is overpressure or leaking. o The pumps do not respond. • The system must alert if: o The charging temperature is higher than 390 ºC. o Some of the electrical heating resistors (electrical oil heater) do not respond. o The cooling-down temperature is higher than custom-set values. Assembly: Parts should be assembled so they are accessible for maintenance purposes. As for the storage, it has got generic requirements (Hermann, 2006): • High energy density (per-unit mass or per-unit volume) in the storage material. • Good heat transfer between heat transfer fluid (HTF) and the storage medium. • Mechanical and chemical stability of storage material. • Chemical compatibility between HTF, heat exchanger and/or storage medium. • Complete reversibility for a large number of charging/discharging cycles. • Low thermal losses. • Easiness of control.

As for more specific requirements, they depend on the storage media type to be used; that is, concrete, a particular PCM or molten salts. In the case of concrete, the main requirements are (ThroughNet, 2008): • Good contact between the concrete and piping. • Good heat transfer rates into and out of the solid medium. Should a PCM be used, the following are required in generic terms (Dinçer, 2002): • The most optimum match between transition zone and operation range. • Highest possible certainty about its long-term thermal behaviour. • Low need of using stabilizing additives. • Low water loss (for encapsulated hydrates). • Suitable melting point at the experiment temperature. • High latent heat of fusion per unit mass. • High density, specific heat and thermal conductivity. • Congruent melting. • Small volume changes during phase transition. • Little supercooling during freezing. • Chemical stability. • No susceptibility to chemical decomposition. • Non-corrosive behaviour to construction materials. • Non-toxic, non-flammable and non-explosive. If molten salt is used, its main requirements are: • Low corrosiveness. • Low freezing point. • As low expansion volume as possible (Rapp, 2005). 3. PLANT DESCRIPTION The heat is transferred to the storage by using thermal oil as heat transfer fluid (HTF). It is initially heated and later pumped towards storage in order to give up the gained heat, and later returned to the heating sub-system. This action is performed cyclically. As just mentioned, the plant features a heating sub-system, which was provided by the company PIROBLOC and manufactured on request according to the exposed requirements in the previous section. It is formed by an electrical heater that works with thermal oil (type CE22 HT), suited for a 22 kW heat capacity and equipped with additional valves should an extra storage sub-system be connected. The most important equipment constructive details are the following: • Heater body built with carbon steel. • Low density 22 kW electrical resistors (0.5 W/cm2) to prevent oil from degrading due to high film temperatures. • A thermal oil pump working under the following parameters: o Flow range: from 3 m3/h to 14 m3/h. o Maximum operation temperature: 400 ºC o Normal operation temperature: 393 ºC

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o Minimum temperature: 25 ºC A cast steel safety and relief valve with a maximum operating pressure of 20 bars. Five stainless steel-made shutoff cocks. Drawn steel pipes and accessories. Pipe thermal insulation (glass wool) up to an 80 mm thickness. A 100 L expansion deposit pressurized with nitrogen (accessories included). A thermal oil cooling battery made with drawn boiler steel.

The oil heater model is shown in Figure 1 (PIROBLOC, 2006). Figure 1. Thermal oil heater manufactured by PIROBLOC.

4. LATENT HEAT STORAGE Initial experiments in the plant are scheduled to be performed by testing latent heat storage on a PCM, whose general requirements have been addressed previously. It is initially desired to work with a material whose phase change takes place in the 160-200 ºC range. The tests goal is headed towards application in a double effect refrigeration installation. In order to carry this out, the following proposed initial conditions are to be set up: • The chosen PCM is to have a melting temperature in the range 170-180 ºC. • The storage capacity to be worked with is to be that of 15 kW. • Heat is to be stored for a one-hour time. It is also expected to work with sensible storage in a near future; both with solid and liquid materials, as implicated in section 3. Observations and results of both types of experiments are to be analyzed and published as well. 5. CONCLUSIONS The plant described in this paper allows designing and characterizing both medium and high temperature TES. All equipment and control devices are already commercially available. The heat transfer media features condition the whole system scope according to the test taking place. Testing of this plant with different storage materials determines both the plant and the respective storage material response to design conditions. As for the use and response of the control system, they are expected to vary with every storage material.

6. ACKNOWLEDGEMENTS The work was partially funded with the project ENE2008-06687-C02-01/CON. The authors would like the company Gas Natural for supporting this work. Dr. Marc Medrano would like to thank the Spanish Ministry of Education and Science for his Ramon y Cajal research appointment. 7. REFERENCES Arce P., Thermal Energy Storage in Medium Temperature Systems. Master Final Project, University of Lleida. 2008. Dinçer I., and Rosen M. A. John Wiley & sons LTD, Chichester,England. Thermal energy storage, systems and applications. Book, Whole. 2002. Herrmann U., Geyer M., and Kearney D.. Overview on thermal storage systems. Report. 2006. PIROBLOC. Calderas de fluído térmico. calderas eléctricas. Personal Communication. Catalogue, 2006. Rapp B. Molten salts. Journal ArticleMater.Today8, 6. 2005. TroughNet. Parabolic Trough Solar Power Network. Parabolic trough thermal energy storage technology. Web Page. 2008.