http://www.lmrabicycleclub.com/gallery/v/club+pics/sun-hot-cold-solar-photo-drawing1024x766.jpg.html?g2_imageViewsIndex=1&g2_GALLERYSID=990f7899d9c3634da87cc3fa52728ece

Kylteknik (”KYL”)

Refrigeration course # 424503.0

9.

v. 2014

Solar cooling (Sustainable cooling) Martin Fält & Ron Zevenhoven Åbo Akademi University Thermal and Flow Engineering Laboratory / Värme- och strömningsteknik tel. 3223 ; [email protected]

Åbo Akademi Univ - Thermal and Flow Engineering

Piispankatu 8, 20500 Turku

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Alternatives 1. Solar cooling – Photovoltaic or thermal 2. Radiative cooling – Heat is radiated to colder air masses situated above 3. District cooling – Centralized production of cooling 4. Passive solutions – The best cooling is one that is not needed

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Solar (heating and) cooling roadmap ~1/6 of energy needs covered by solar energy by 2050

Source: http://www.iea.org/publications/freepublications/publication/Solar_Heating_Cooling_Road map_2012_WEB.pdf 3.12.2014

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Solar (heating and) cooling roadmap current costs vs. electricity / gas

Source: http://www.iea.org/publications/freepublications/publication/Solar_Heating_Cooling_Road map_2012_WEB.pdf 3.12.2014

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9.1 Solar cooling

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9.1 Solar cooling  Cooling demand is in phase with incident solar energy  15% of the world's electricity (> 20% in some countries) is used for refrigeration and air-conditioning  Electricity price- and load-peaks are common when cooling demand is high

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Radiation from the sun /1  Consider the sun as a blackbody radiator at 5800 K. The diameter of the sun is ~ 1.39×109 m, the diameter of earth is ~ 1.29×107 m; the distance between earth and sun is ~ 1.5×1011 m. 1.

The solar radiation output equals Qsun = π· R2s· σ· T4sun = 3.89×1026 W

2.

The fraction of Qsun that is intercepted by earth equals Qsun-earth = Qsun × (π/4)· R2earth/(4π·S2earth-sun) = Qsun × R2earth/(16·S2earth-sun) = 1.80×1017 W

3.

Heat flux to spherical earth equals Q”sun-earth = Qsun /(4πS2earth-sun)= 1377 W/m2 As the earth’s trajectory is elliptical a correction equation exists:  d  W  Q  13771  0.033  cos 2 ,   2  365   m 

d  day number

Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, 20500 Turku

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Q   Q0  cos 

 The incident heat flux on a surface is also determined by its orientation  Which can be calculated with Lamberts Law  This introduces cos(θ), which in turn is dependent on location and time

http://www.brighton-webs.co.uk/energy/solar_earth_sun.htm

Radiation from the sun /2

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Radiation from the sun /3

 cos sin sin  sin 

http://www.brighton-webs.co.uk/energy/solar_earth_sun.htm

cos    sin  sin  cos  sin  cos sin cos  cos cos cos cos  cos cos sin cos cos

 ϕ the latitude  δ the solar declination  ω hour angle  ν the surface angle vs. horizontal  γ the azimuth angle of the plane  θ the radiation angle vs. the plane normal TL10

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 Turku has the latitude 60°30’.  Solar declination δ :  

  23,45  sin  2

284  n   365 

 ω hour angle eg. at 14:30 ω = -37.5°  For horizontal surface ν = 0°  For vertical surface ν = 90°  South side surface γ

http://www.itacanet.org/the-sun-as-a-source-of-energy/part-1-solar-astronomy/

Radiation from the sun /4

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 The transmittance through the atmosphere decreases the 1 AM (air mass)  1.6364 intensity of the cos   0.50572 96.07995   sun becomes  2 for θ  ~ 60  This is due to direct solar irradiation gases and particles 0.678 that absorb heat Id  1377  0.7 AM  W/m 2 radiation Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, 20500 Turku

http://www.brighton-webs.co.uk/energy/solar_earth_sun.aspx

Radiation from the sun /5

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9.1.1 Photovoltaic refrigeration

Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, 20500 Turku

Solar PV /1

Pic: http://www.maproyalty.com/solar.html

 Photo-voltaic energy is based on radiation creating electric potential differences, ∆V, that can generate electric current, i, making use of semi-conductors (in which electrons can occupy ranges (”bands”) of energy levels).  This gives a power outputs up to 100 - 200 W/m2, 0.1 – 0.25 kW/panel  Typical system size 1 – 100 kW; largest PV plants are > 250 MW.  Most units are based on crystalline silicon (c-Si) but amorphous materials take over?  In 2005, ~ 3 TWh (3×106 kWh) (IEA08) solar electricity was generated, ~ 105 TWh in 2012  Total installed capacity 20102013: 25~100 GW Åbo Akademi Univ - Thermal and Flow Engineering

Solar PV /2

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Shockley–Queisser limit for p-n junction PV power

 Solar energy efficiency η, defined as the ratio electric power out/solar energy absorbed: η = ∆V·i/Qin

Åbo Akademi Univ - Thermal and Flow Engineering

Pic: http://www.greentechnolog.com/2007/12/ largest_solar_photovoltaic_array_is_active_at_air.html

 Is found to be ≤ 33.7% for non-concentrated sunlight; (Si: 1.1 eV ~29 %) with concentration factors C ~ 400 values up to η ~ 35% are reached with bandgap voltages ~1.1 – 1.3 eV. In practice η > ~ 0.15 is difficult.

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Pic: http://www.geocities.com/Eureka/1905/_pvFinland.gif

(with ∆V ~ 0.5 V and i ~ 3 A per cell)

14

Solar PV /3

Sun & Wind Energy 2012/02 p. 90 Åbo Akademi Univ - Thermal and Flow Engineering

Piispankatu 8, 20500 Turku

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Theory to practice  Connect a photovoltaic system with a refrigeration system  Evaluate the need for storage energy  Easy to implement in existing systems  Refrigeration system: • • • • • •

Vapor compression Thermoelectric element Stirling refrigerator Thermo-acoustic refrigeration Magnetic cooling Electrochemical refrigeration 3.12.2014

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15

Solar thermal     

Connect a cooling system to a solar collector Flat-plate solar collector or evacuated collector Flat-plate collectors are cheaper but give lower temperatures Evacuated tubes give higher temperatures but more expensive By using reflectors an increase is attainable nonetheless it is not suitable for all regions

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Thermo-mechanical refrigeration  Solar heat → Mechanical work → Vapor compression  Rankine and Stirling* cycle engines are being considered  Expensive when compared with photovoltaic refrigeration  Theory: – Müser engine – Stefan-Boltzmann engine * See Low temperature differential (LTD) Stirling engine: http://diystirlingengine.com/ltd-stirlingengine/ Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, 20500 Turku

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Thermo-mechanical refrigeration  Ideal Carnot System efficiency  Tg is the temperature of the fluid in the solar collector  T1 is the ambient temperature  T2 is the refrigerated temperature

Carnot Heat Engine  1

T1 Tg

COPcarnot  1 

T2 T1

carnot   carnot Heat Engine  COPcarnot   

carnot  1 

T1 Tg

 T2  1    T  1  

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Sorption refrigeration  Open or closed systems  Two different closed-sorption techniques: absorption and adsorption: – Absorption - one substance is “sucked up “ by (into) another – Adsorption – one substance forms a surface on the other surface

 Two different open systems: liquid and solid Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, 20500 Turku

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Sorption refrigeration 1. Generator receives heat to regenerate the sorbent that has absorbed the refrigerant 2. The refrigerant vapor is condensed and rejects heat to the ambient 3. The regenerated sorbent is sent to the absorber 4. The condensed refrigerant is evaporated while cooling 5. The regenerated sorbent “absorbs” the refrigerant and rejects the sorption heat to the ambient Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, 20500 Turku

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Absorption  Most common solar refrigeration system  Generation temperature below 100°C  Ammonium/water and water/lithium bromide are the most common refrigerantabsorbent pairs (see course #3)  The COP of a single-stage is around 0.6 - 0.8  The COP can be increased by adding stages but this increases the costs  Physically smaller than adsorption

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Absorption

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Adsorption  Involves a solid sorbent that attracts solid refrigerants on its surface chemically or physically  The physical mechanism is the Van der Waals force  Chemical adsorption requires more energy to remove the refrigerant than physical adsorption, covalent or ionic bond  Most commonly used chemicals are CaCl2 which absorbs ammonia and water to produce CaCl2.8NH3 and CaCl2.6H2O  Low cooling power density → big systems Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, 20500 Turku

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Adsorption

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Desiccant cooling /1  Also called open sorption cooling  The sorbent is used to dehumidify air  All water adsorbents coolers can be used as desiccant coolers  Two main designs: solid and liquid  In the liquid design the liquid is pumped between the regenerator and dehumidifier  In the solid there exist solid wheels that do the same work  In theory the same COP as a closed adsorption cooler  It is a HVAC (Heat, Ventilation and Air Conditioning) unit  Perfect for a large need for ventilation and dehumidification Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, 20500 Turku

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Desiccant cooling /2 humidity control

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Liquid desiccant cooling

A→B The solution absorbs moisture from the air and is cooled down by the cooling coil C→D The cooled solution is reheated → higher COP D →E The water in the solution is desorbed

1→2 2 →3 4 →5

Ambient temperature is cooled and dehumidified After-cooler is optional Ambient air is humidified

Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, 20500 Turku

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Pic: http://sustainabilityworkshop.autodesk.com/buildings/humidity-control

Solid desiccant cooling

Dessicant wheel A→B B →C C →D D →E

Evaporative cooling 1→2 reduces temperature Wheel is cooled and 2 →3 Temperature increased by the 4 →5 solar coil Hot & humid air regenerates the desiccant wheel

Ambient air is dehumidified Air is cooled Aftercooler is optional DK08

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Comparing the techniques 20°C

300°C Concentrating

heat Solar Thermal Collectors -Flat Plate -Evacuated Tube -Concentrating

Air-conditioning

15°C

150°C ETC 100°C Flat Plate 70°C

8°C Food, Vaccine Storage

0°C Freezing

WP06 3.12.2014

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Comparing the techniques

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Use of solardriven cooling

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Comparing usage

DEC= Desiccant cooling 3.12.2014

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Comparing usage

ECT= CPC= Air= PTC=

Evacuated Tube Collector Compound Parabolic Concentrator Air collector Parabolic-trough collector 3.12.2014

Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, 20500 Turku

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Simulating solar cooling systems  Use as little of electrical energy as possible  Avoid electrical peak loads  Lower operation costs TDC = Thermal Driven Cooling, Chilling

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     

Collector size? Storage? Heat rejection? Cooling type? Backup? Control of system?

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9.2 Radiative cooling

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Earth-Atmosphere Energy Balance

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Radiative cooling - Gray assumption

Atmosphere

Sky

ܳሶ ൌ ߝ௥௔ௗ ߝ௦௞௬ ߬௔௧௠ ߪ ܶ௥௔ௗ ସ െ ܶ௦௞௬ ସ

Radiator

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http://www.srh.noaa.gov/jetstream//atmos/a tmprofile.htm

Temperature profile atmosphere

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Temperature of sky?  The sky temperature can be measured with a pyrgeometer.  It measures the energy flow in the interval 4.5-42 µm as a single value  Here the ambient and the sky temperatures are plotted ସ ସ  = ௐ ௠మ Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, 20500 Turku

Radiative cooling

Qrad(Tsky) Qconv (Tamb)

 Passive/natural cooling  Natural: utilizing the variations of ambient by convection and sky temperature by radiation  Storing cold at night for cooling purposes during the day

Roof heat exchanger H2Oin

H2Oout Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, 20500 Turku

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Example  A recent article (Dimoudi and Androutsopoulos, 2006) discussed “The cooling performance of a radiator based roof component”

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Atmospheric Window 7.9-13µm

Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, 20500 Turku

http://chriscolose.wordpress.com/2010/02/ 18/greenhouse-effect-revisited/

Radiation Transmitted by the Atmosphere

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Possibility of wavelength dependency?

Atmosphere

Tsky

Tsky

Tsky Atmosph -eric window

Trad

Trad 7.9 µm

Trad 13 µm

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Possibility of wavelength dependency?

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HEAT

Skylight for passive cooling against the sky

 Recent work at ÅA VST “Thermal radiation heat transfer: including wavelength dependencies into modeling” Zevenhoven, Fält, Gomes, L.P. Int. J. of Thermal Sciences 86 (2014) 189-197 “Combining the radiative, conductive and convective heat flows in and around a skylight” Fält, Zevenhoven, J. of Energy and Power Engineering 6 (2012) 1423-1428 Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, 20500 Turku

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Skylight filled with air - cooling Heat flux 15 W/m2. Simulated with Comsol 4.1 Velocity profile in cm/s

Temperature profile in °C

0.5 m

0.1 m

Weather data Helsinki, July 2008 Åbo Akademi University |Thermal and Flow Engineering | 20500 Turku

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Skylight filled with CO2 - cooling Heat flux 117 W/m2. Simulated with Comsol 4.1 Velocity profile in cm/s

Temperature profile in °C

0.5 m

0.1 m

Weather data Helsinki, July 2008 Åbo Akademi University |Thermal and Flow Engineering | 20500 Turku

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Possibility of wavelength dependency!  By using resistance networks one can calculate the impact of wavelengths  4 wave-length intervals  Properties of the radiator are changed  Same temperatures give μm ௐ 49 ௠మ  0°C gives 19

ௐ

௠మ

0-4

4-8

8-14

14-∞

ρ

0,95

0,95

0,05

0,95

ε

0,05

0,05

0,95

0,05

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9.3 District cooling

Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, 20500 Turku

 Cooling is produced centrally  Typical cooling objects are the ventilation in large office buildings

Finland

 The cooling is produced by four main methods  Popular in Finnish cities: ~95 km total, ~ 200 MW  Paris has the largest district cooling capacity 380MW

http://energia.fi/en/slides/district-cooling-graphs-year-2013

District cooling

TE09 CS11

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District cooling  Advantages are: low energy consumption, small installations at sight, no health problems as legionella

 Turku has a district cooling net with a capacity of 14 MW  Heat pump that uses the sewage water

TE09 CS11 Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, 20500 Turku

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9.4 Passive solutions

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http://www.eslarp.uiuc.edu/arch/ARCH371-F99/groups/k/solar.html

9.4 Passive solutions

Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, 20500 Turku

            

Source #09

A11: R. C. Arora ”Refrigeration and air conditioning”, 2nd. Ed. PHI Learning Private Limited , New Delhi (2011) BP12: BP Statistical Review of World Energy June 2012, 37 Renewable energy WP06: W. Pridasawas ”Solar-Driven Refrigeration System with Focus on the Ejector Cycle” KTH (2006) WS09: W. Sparber, A. Napolitano, G. Eckert A. Preisler ”State of the art on existing solar heating and cooling systems” IEA SHC Task 38 Solar Air conditioning and Refrigeration (2009) DK08: D.S. Kim C.A. Infante Ferreira ”Solar refrigeration options ̶ a state-of-the-art review” Int. J. Refrig. 31 (2008) 3-15 KF09: K.F. Fong “Comparative study of different solar cooling systems for buildings in subtropical city” Sol. Energy 84 (2009) 227-244 DM12: D. Mugnier, J Uli “Keeping cool with the sun” (2012) ISER 28-30 TL10: T. Lönnroth ”Emerging Nuclear Energy Systems I 3rd Edition” ÅA, 2010 RZ13: R. Zevenhoven ”Process Engineering thermodynamics” Chap. 2b, (2013) RZ14: R. Zevenhoven ”Introduction to Process engineering” (PTG) chap. 5, (2014) GA10: Ghoniem, Ahmed F. “Needs, resources and climate change: Clean and efficient conversion technologies” Prog. Energy Combust. Sci. 37 (2009) 15-51 CS11: Climespace GDF SVEZ “Discover District Cooling” (2011) TE09: Åbo Energi “Kakolan lämpöpumppulaitos” Ekologista kaukolämpöäa ja kaukokylmää turkulaisille (2009) http://www.turkuenergia.fi/files/8713/7034/5690/Kakolan_lampopumppulaitos.pdf Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, 20500 Turku

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End of course v. 2014 Åbo Akademi Univ - Thermal and Flow Engineering - Piispankatu 8, 20500 Turku