Solar-Cooling Technologies PDF

Renewable energy technologies Solar-Cooling Dipartimento di Ingegneria Industriale Università di Padova SOLAR COOLING Absorption cycle  SOLAR-COOLING Solar-cooling technologies are based on the use of solar energy for the "producti...

Renewable energy technologies Solar-Cooling Dipartimento di Ingegneria Industriale Università di Padova SOLAR COOLING Absorption cycle  SOLAR-COOLING Solar-cooling technologies are based on the use of solar energy for the "production of cold" (refrigeration, air conditioning, food conservation). Example of figure: refrigeration system with heat input (absorber) + solar collectors. Heat rejected Solar collectors Absorber Cold Hot storage storage User  SOLAR-COOLING Pannelli solari PHOTOVOLTAIC fotovoltaici MODULES ELECTRIC SOLARE SOLAR ELETTRICO COOLING AND Raffrescamento e DEHUMIDIFICATION deumidificazione Refrigeratore elettrico Accumulo (a compressione) elettrico (batterie) Pannelli solari THERMAL COLLECTORS termici SOLARE TERMICO THERMAL SOLAR Raffrescamento COOLING ANDe/o deumidificazione DEHUMIDIFICATION Refrigeratore termico: Assorbimento Accumulo Adsorbimento termico Essiccante liquido Essiccante solido  SOLAR-COOLING PV Pannello MODULE solare Compressore COMPRESSOR Condensatore Motore ENGINE Evaporatore VAPOR Gruppo refrigeratore a COMPRESSION compressione REFRIGERATOR Traditional electrically powered chiller with photovoltaic modules (solar-electric cooling is not often considered in the "solar-cooling" category)  SOLAR-COOLING Advantages: - strong reduction in electricity consumption in the air conditioning of buildings - strong reduction of pollutant emissions (NOx, SO2) and greenhouse gases (CO2) - overlap between solar radiation availability and cooling demand - large surfaces of solar collectors (required for solar-cooling) can make a significant contribution even in winter for space heating / DHW Critical issues: - installation costs (higher than traditional systems) - heat rejection is a critical point - lack of knowledge of technology - non self-sufficiency of the solar cooling system  SOLAR-COOLING Heat rejected Solar collectors Absorber Cold Hot storage storage User SINGLE EFFECT ABSORPTION REFRIGERATING EQUIPMENT Q0  Absorption cycle: max ideal COP 1° Law Th. 2° Law Th.  Absorption cycle: max ideal COP Let us divide each term by qg  Absorption cycle: max ideal COP When Tg --> ∞ If COPC=7,5 SOLAR-COOLING Refrigerating Absorption systems Refrigerating Adsorption systems DEC (Desiccant Evaporative Cooling) Systems SORPTION TECNOLOGIES - absorption is the process in which a substance in one phase is incorporated into another substance of a different phase (e.g. gases being absorbed by a liquid); - adsorption refers to the use of a solid for adhering or bonding ions and molecules of another substance onto its surface; Besides absorption and adsorption refrigeration, desiccant cooling is a new kind of thermally activated technology The process of adsorption concerns separation of a substance from one phase, accompanied by its accumulation or concentration on the surface of another. Both absorption and adsorption are used to provide thermal compression of the refrigerant instead of mechanical compression. Evaporation and sorption of a refrigerant generates a useful cooling effect in the evaporator.  SOLAR-COOLING Solar-cooling systems based on absorption chillers operate as closed-circuit systems: they produce cold / chilled water. They can be combined with any type of distribution system (air treatment unit, fan coil, radiant panels, etc.). Absorption chillers can be single effect (thermally fed at low temperature, 170 ° C, with COP 1-1.2). They work with a couple of substances (absorbent/refrigerant): water/ammonia, LiBr/water, LiCl/water. Thermally activated cooling technologies Absorption cycle Characteristics of lithium bromide-water absorption systems  Coefficient of performance of absorption chillers 3 effects 2 effects 1 effect Chilled water: 7 °C Cooling water: 30 °C copertura vetro assorbitori tubi vetro evacuati canali assorbitori isolamento canali 50 125 200 275 350 Heat supply temperature [°C]  SOLAR-COOLING Adsorption chillers Solar cooling systems based on adsorption chillers operate as closed circuit systems: they are used to produce cold water (as in absorbers). The adsorbers use desiccant materials: the physical principle used is similar to that of DEC systems. Adsorption chillers have lower COP (0.5-0.55) than absorbers, but can be powered at lower temperatures (55-65°C).  Materials for adsorption chillers SILICA GEL It is used as a desiccant and for local humidity control. The chemical-physical process by which the water vapor is removed from the air consists of an adsorption operation: the water vapor molecules bind to the surface of the silica gel, which must be understood not only as an external surface, but also as a surface in full; the silica gel in fact has a considerable porosity, and this translates into a greater total area available for the exchange of matter, or rather a fairly high speed of the dehumidification process.  ADSORPTION CHILLER Condenser Heat exchangers Cooling water 1 and 2 circuit Silica Gel matrix Heating water circuit Chilled water circuit At t=45 °C p=0,096 bar Evaporator At t=5 °C p=0,0087 bar  Desiccant cooling (DEC) systems The system known as "desiccant cooling", or cooling with desiccants, is used for summer air conditioning of indoor environments. The system is based on an open cycle operating with humid air. One of the advantages of the system is that of being able to achieve a refrigeration effect without the need to use refrigerants. Since the system requires a thermal input from the outside, one can think of using part of the heat discharged from an upstream heat source. A system of this type consists of an external air inlet line to the room to be conditioned and a second outlet line. The two lines are not completely separated: an exchange of heat is provided by means of an internal heat exchanger which transfers heat from the incoming to the outgoing air, and an exchange of humidity by means of a hygroscopic dehumidifier (with "silica gel" chemicals); the latter lowers the humidity of the incoming current, removing water, and increases the humidity of the outgoing air, giving to it the water removed from the incoming air current. Adsorption system with dessicant cooling  Desiccant cooling (DEC) systems The inlet line allows the entry of external air, which enters a dehumidifier, where it is initially dehumidified and heated. Then the air passes into a heat exchanger which cools it without changing its moisture content. Subsequently, humidification is carried out with an evaporative system which also lowers the air temperature. The indoor air to be conditioned is taken and sent to the outlet line, where evaporative cooling occurs first; the air is subsequently heated by the heat exchanger which uses the heat contained in the external air; further heating of the air takes place by means of an exchanger which is fed with heat from the external heat source. Finally, the air, before being expelled into the external environment, passes through the hygroscopic dehumidifier which transfers further water to the hot air stream. The heating of the outgoing air with the air heater (external heat source) is necessary to reduce the relative humidity of the incoming air, which in this way is able to absorb a greater amount of water.  SOLAR-COOLING collettori solari raffreddatore evaporativo espulsione aria di ripresa batteria di scambio termico aria di immissione ripresa ruota di scambio ruota essiccante postraffreddatore termico Dehumidification with solid dessiccant  SOLAR-COOLING DEC Systems – Desiccant Evaporative Cooling DEC systems are open circuit systems: they are used to directly treat / condition the air taken from the external environment. They combine dehumidification and evaporative cooling processes. They are applied to AHUs (air handling units – UTA in Italian) which require an adequate air distribution system in the building.  AIR DEHUMIDIFICATION WITH SOLID DESSICANT RIGENERATION AIR FROM WHEEL RIGENERATION AIR HEAT OUTSIDE DRY AIR FOR BUILDING HUMID AIR VENTILATION