14 ASHRAE Journal ashrae.org June 2006 June 2006 ASHRAE Journal 15 Solar฀cooling฀has฀a฀strong฀potential฀for฀signifi ฀cant฀primary฀energy฀savings. About the Authors Constantinos A. Balaras, Ph.D., Group Energy Conservation, Institute for Environmental Research & Sustainable Development, at the National Obser- vatory of Athens, Greece; Hans-Martin Henning, Ph.D., and Edo Wiemken, Thermal Systems and Buildings, Fraunhofer Institute for Solar Energy Systems, Freiburg, Germany; Gershon Gross- man, Dr.Sci., professor of mechanical engineer- ing, Technion-Israel Institute of Technology, Haifa, Israel; Erich Podesser, Ph.D., Joanneum Research, Institute of Energy Research, Graz, Austria; Carlos A. Infante Ferreira, Ph.D., Delft University of Technology, Delft, The Netherlands. S olar air conditioning is an emerging market with a huge growth potential. Peak cooling demand in summer is associated with high availability of solar radiation, which offers an excellent opportunity to exploit solar energy with heat-driven cooling machines. The main obstacles for large-scale applications, besides high first cost, are the lack of practical experience with the design, control, operation, installation and maintenance of these systems. * Defined as the ratio of the cooling capacity of the system and the heating power delivered to the system by the solar collectors—directly or indirectly through the storage vessel. By Constantinos A. Balaras, Ph.D., Member ASHRAE, Hans-Martin Henning, Ph.D., Edo Wiemken, Gershon Grossman, Dr.Sci., Erich Podesser, Ph.D., and Carlos A. Infante Ferreira, Ph.D. For฀low฀power฀cooling฀systems,฀com- mercial฀technologies฀are฀available฀on฀a฀ limited฀basis.฀However,฀a฀strong฀focus฀ exists฀for฀research฀on฀other฀applications฀ including฀photovoltaic-operated฀refrig- eration฀ cycles฀ and฀ solar฀ mechanical฀ refrigeration. 1 This฀article฀reviews฀representative฀Eu- ropean฀solar฀cooling฀installations฀and฀pro- vides฀some฀practical฀design฀guidelines. The฀ main฀ heat-driven฀ cooling฀ tech- nologies฀include: • Closed-cycle฀ systems.฀Absorption 2 and฀adsorption 3 ฀cycles฀are฀examples.฀ They฀produce฀chilled฀water฀that฀can be฀used฀in฀combination฀with฀any฀air- conditioning฀equipment฀such฀as฀an฀ air-handling฀unit,฀fan-coil฀systems, chilled฀ceilings,฀etc. • ฀ Open-cycle฀ systems .฀Desiccant฀ systems฀are฀one฀example. 3 ฀The฀term฀ “open”฀cycle฀is฀used฀to฀indicate฀that฀ the฀refrigerant฀is฀discarded฀from฀the system฀after฀providing฀the฀cooling฀ef- fect,฀and฀new฀refrigerant฀is฀supplied฀in฀ its฀place฀in฀an฀open-ended฀loop.฀ Comparing Technologies Single-฀and฀double-effect฀gas-fi ฀red฀ab- sorption฀systems,฀primarily฀manufactured฀ in฀the฀U.S.,฀China,฀Japan,฀and฀India,฀are proven฀and฀marketable฀technologies฀that฀ may฀be฀adapted฀and฀integrated฀in฀a฀solar- assisted฀installation.฀Typical฀coeffi ฀cient฀of฀ performance฀(COP)*฀for฀large฀single-ef- fect฀machines฀are฀0.7฀to฀0.8.฀For฀improved฀ Solar Cooling An Overview of European Applications & Design Guidelines performance,฀double-effect฀absorption฀systems฀are available,฀with฀typical฀operating฀COPs฀of฀1.0฀to฀1.2.฀ Current฀research฀is฀focusing฀on฀three-฀and฀four-effect฀ systems,฀ which฀ present฀ an฀ attractive฀ potential฀ for฀ improved฀cooling฀performance,฀with฀a฀COP฀of฀1.7฀ to฀2.2. 4 For฀solar-assisted฀systems,฀it฀is฀important฀to฀ select฀the฀appropriate฀solar฀collector฀type฀to฀meet฀the temperature฀needs฀of฀the฀cooling฀machine.฀Systems with฀high฀COP฀values฀need฀higher฀operating฀tem- peratures.฀ For฀ example,฀ double-effect฀ cycles฀ need฀ high-effi ฀ciency฀concentrating฀collectors. Commercially฀available฀absorption฀chillers฀range฀ in฀capacity฀from฀medium฀(40฀to฀100฀kW฀[11฀to฀28฀ tons])฀to฀high฀(300฀kW฀[85฀tons]฀and฀up).฀A฀number฀ of฀large-scale฀solar฀cooling฀systems฀for฀large฀com- mercial฀building฀and฀industrial฀applications฀have฀been฀ successfully฀demonstrated฀and฀it฀is฀now฀necessary฀to support฀wide฀market฀introduction.฀However,฀given฀the฀ increasing฀cooling฀demand฀in฀residential฀and฀small฀ size฀building฀applications,฀a฀growing฀market฀exists฀for฀ low฀cooling฀capacity฀equipment฀(i.e.฀less฀than฀10฀kW฀ [34,000฀ Btu/h]).฀ Small-scale฀ solar฀ cooling฀ systems฀ could฀be฀ready฀within฀a฀decade฀or฀earlier,฀if฀research฀ and฀development฀support฀is฀provided. Today, adsorption฀chillers฀have฀a฀higher฀effi ฀ciency฀ than฀absorption฀chillers฀at฀low฀driving฀temperatures฀(defi ฀ned฀as the฀average฀temperature฀of฀the฀heating฀fl ฀uid฀between฀inlet฀and฀ outlet฀of฀the฀heating฀system).฀The฀advantage฀is฀that฀their฀internal฀ cycle฀does฀not฀have฀any฀moving฀parts฀(no฀pumps,฀no฀electrically฀ driven฀valves).฀Also,฀crystallization฀cannot฀occur,฀as฀in฀the฀case฀of฀ LiBr/H 2 O฀absorption฀chillers.฀However,฀due฀to฀their฀intermittent operation฀ (periodic฀ cycle),฀ they฀ require฀ more฀ effort฀ in฀ system฀ design฀and฀operation฀control.฀In฀addition,฀compared฀to฀absorption฀ machines,฀they฀are฀larger,฀heavier,฀and฀more฀expensive฀per฀kW฀ cooling฀capacity.฀Only฀a฀few฀manufacturers฀make฀the฀systems,฀ limiting฀equipment฀choices.฀The฀COP฀of฀commercially฀available฀ systems฀is฀0.55฀to฀0.65,฀depending฀on฀operating฀conditions.฀ The฀use฀of฀sorption฀air฀dehumidifi ฀cation,฀using฀solid฀or฀liquid desiccants,฀offers฀an฀alternative฀to฀conventional฀vapor฀compres- sion฀equipment.฀When฀combined,฀they฀lead฀to฀higher฀effi ฀ciency฀ by฀increasing฀the฀evaporator฀temperature฀of฀the฀compression cycle.฀Most฀desiccant฀systems฀presently฀use฀a฀solid฀desiccant material฀ like฀ silica฀ gel.฀ Since฀ the฀ solid฀ desiccant฀ cannot฀ be pumped,฀these฀systems฀usually฀use฀a฀desiccant฀wheel฀to฀allow continuous฀operation.฀The฀use฀of฀solid฀desiccants฀makes฀sense,฀ if฀the฀air฀change฀and/or฀the฀dehumidifi ฀cation฀of฀the฀indoor฀air฀ are฀necessary฀or฀strictly฀prescribed.฀Examples฀exist฀in฀super- markets,฀museums,฀and฀assembly฀halls฀with฀high฀occupancy. Systems฀using liquid฀desiccants฀are฀less฀widespread.฀The liquid฀desiccant฀system฀essentially฀is฀an฀open-cycle฀absorption฀ system,฀where฀water฀serves฀as฀the฀refrigerant.฀It฀has฀various฀ advantages฀including:฀ •฀ Fewer฀components฀since฀condensation฀of฀the฀refrigerant฀no longer฀uses฀a฀condenser฀but฀rather฀uses฀the฀environment; •฀ The฀whole฀system฀operates฀at฀atmospheric฀pressure,฀thus alleviating฀the฀need฀for฀pressure-sealed฀units; •฀ The฀ amount฀ of฀ refrigerant฀ (water)฀ evaporated฀ in฀ the฀ re- generator฀is฀independent฀of฀an฀evaporator,฀thus฀providing฀ greater฀fl ฀exibility;฀and •฀ More฀effi ฀cient฀use฀of฀low฀heat฀driving฀temperatures฀(down฀ to฀about฀60°C฀to฀70°C฀[140°F฀to฀158°F]).฀ However,฀this฀technology฀has฀not฀yet฀arrived฀in฀the฀market,฀ and฀further฀system฀optimizations฀are฀necessary.฀Commercially฀ available฀systems฀are฀expected฀soon. Desiccant฀air-conditioning฀plants฀need฀a฀good฀control฀system with฀a฀reliable฀control฀strategy,฀which฀can฀ensure฀an฀economi- cal฀year-round฀operation—air฀conditioning฀in฀summer,฀heating and฀humidifi ฀cation฀in฀winter,฀and฀dedicated฀ventilation฀during intermediate฀seasons,฀if฀there฀are฀no฀heating฀or฀cooling฀loads. The฀COP฀of฀desiccant฀systems฀is฀defi ฀ned฀as฀the฀change฀in฀ enthalpy฀of฀the฀process฀air฀(resulting฀from฀reduction฀of฀both humidity฀and฀temperature)฀divided฀by฀the฀heat฀input฀in฀the฀ regenerator.฀The฀COP฀of฀a฀desiccant฀cooling฀system฀strongly฀ © 2006 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Journal (Vol. 48, June 2006). For personal use only. Additional distribution in either paper or digital form is not permitted without ASHRAE’s permission.