Solar Cooling

Overview

Why solar cooling? In many countries, especially in developing countries in the South, with increasing economic development and population growth, demand for cooling is increasing rapidly (e.g. ric Arab Countries - 75% of installed poer is used for sooling). Often, this additional electricity load puts further stress on the mostly already shaky grids in these countries, leading to further power cuts.

Also, in many areas cooling for agricultural products, vaccines, etc. is an essential need which cannot be served. In this case, solar is also on option.

The main arguments for solar assisted cooling (SAC) originate from an energy saving perspective:

• Application of SAC saves electricity and thus conventional primary energy sources
• SAC also leads to a reduction of peak electricity demand this can benefit the electricity network and lead to additional cost savings of the most expensive peak electricity (if applied on a broad scale)
• environmentally sound materials without ozone depletion and no (or very small) global warming potential are used with SAC
• Coincide of solar energy supply and demand in many cases - when it is the hottest and most cooling is demanded, usually the most sun is shining as well.

Applications

Solar assistes cooling can be broadly split up in two main applications, depending on the targeted temperature range:

• air conditioning – temperature range of 5-20°C
• refrigeration – temperature range of -20°C to +5°C

State of the Art world wide Solar Air Conditioning

Despite intensive research over the past decade,SAC has still reached only a very small market penetration. Yet, a well established SAC research society and scientific field are working on further market development.

• Close to 1.000 SAC systems installed worldwide
• Huge variety in sizes and technologies
• Upcoming larger >400kW and very large > 1MW projects
• More and more chiller products in smaller range <25kW
• With around 60% vapor absorption systems (VAM) dominate the market
• Improving system performance
• Early market development
• Small but increasing number of projects by private investors
• Most systems in USA, Germany, Southern Europe and MENA
• Few companies offering complete SAC solutions
• Custom made systems -> acking experience and expertise, very expensive
• Cost reduction expected with increasing standardization, economies of scale and upcoming specialized incentive schemes (e.g. France)
• Could take off in the coming years with increasing energy prices and further experiences in hotter climates (higher irradiation, higher cooling loads)

Major Challenges

A range of challenges exist why solar cooling has not taken off so far. In many cases, it is a combination of different issues.

Technology

Still, most of the issues are related to the technology. One of the main problems beeing that there is not one single solution and experiences with new applications are collected constantly

• Small capacity VAM units under development, expensive
• Few suppliers for adsorption and desiccant systems
• No packaged solutions for residential and small commercial users available
• Lacking standardization of applications, mostly still tailor-made
• No great variety of medium temperature collectors
• Thermal efficiencies often still low
• Heat rejection systems: often wet cooling tower needed
• Lacking professional skills – high skilled labour needed, also for maintenance
• Still electricity for control systems, pumps etc needed – can be significant
• Over-dimensioning should be avoided – otherwise low efficiency
• Heat rejection systems need a lot of electricity and maintenance
• Auxiliary components not energy efficient
• Maximizing usage (hot water, heating and cooling) important
• System integration and energy management inadequate – combination of subsystems and skills
• Experience in design, control and system operation lacking

Cost

• High capital investment costs (2-5 times) compared to conventional systems
• Price range: EUR 2.000-5.000/kW_cold
• Payback periods of <10 years under promising conditions possible (< 7 years have already been reached by some systems)
• Often high maintenance costs - depending on technology
• Unfavorable finance environment
• Financial incentive scheme not explicitly designed to fulfill special SAC needs, often same as solar thermal for heating, if any

Policy

• Cooling mostly not yet part of policy target and strategies
• Regulatory measures needed

Awareness

• Lacking knowledge and practical experience of architects, planners and builders
• Lacking (large-scale) experiences and showcases for replication

Solar Thermal vs. Photovoltaic (PV)?

New discussion due to decreasing PV prices. Could it be more economical to run vapour compression chiller with a PV module than operating solar thermally powered chillers?

The discussion is still going on, there is no clear answer yet available to this question as the answer depends o a range of different thinkable boundary:

• First examples of competitive installations – e.g. Cyprus (very high electricity price)
• Maybe suitable alternative in some cases
• Depending on boundary conditions and objective of cooling – very site specific
• Availability and reliability of grid as storage - if needed
• Storage/ back-up issue
• alternative energy costs (esp. electricity)
• Subsidies and incentives available for conventional power/systems/energy switch, etc.
• Different effect on grids - what do I want to achieve with solar cooling?
• Demand for other energy services – demand for hot water and heating
• Still discussion and research going on in solar cooling community
• Solar thermal and chiller costs expected to decrease with market growth
• Upcoming support schemes for solar thermal cooling
• lower efficiency of PV modules in hot climates but lower maintenance (e.g.)
• Energetically PV less ideal than thermal

Country Experiences

Solar Cooling in India

In India, boundary conditions for solar cooling are very favourable: there is more irradiation and at the same time more cooling degree days than in areas where most systems so far have been installed (e.g. USA, MENA, South Europa) - but partly in some regions the climate is also more humid which asks for adapted systems

Ideal boundary conditions (high solar radioation, long cooling season). The only drawback are not prohibitive high energy price, yet this is neutralized trough extensive electricity shortages. Many institutions, companies etc. have their own back up systems for electrcity supply, often also for cooling.

Indian customers are looking for reliable cooling options - this is a chance for solar applications.

Moreover, the cooling demand is growing further which is endangering grid stability even more

The total installed cooling load is 35.000MWe (28.7% of installed capacity)

Residential sector – great future challenge

• Penetration level for A/C < 1% only and switch from air evaporation systems expected
• Lacking affordabel, small scale renewable technologies
• Highest load in non-sunshine hours -> no coincide of supply and demand, challenge of storage

Industrial and commercial sector – short and medium term market

• Larger central systems in place already – solar technologies available for these sizes
• often coincide of loads and solar gains

Solar Cold Storage

• India is looking for cold storages for a range of applications: fishing, agricultural produce, milk and dairy products etc. as vast part of rural areas still lack access to grid electricity. Currently TERI is working on a combined solution, a http://www.inive.org/members_area/medias/pdf/inive/clima2000/1997/p301.pdf biomas-solar-hybrid-electricity grid with cold storage, yet this project is still at a pilot phase.

Experiences

The Ministry of New and Renewable Energe (MNRE) proposed an Action Plan on Solar Coolingin 2009, together with the establishment of a working group on solar cooling. However, the plan has not been followed. There are no updates and the working group is not active anymore.

Because of electricity shortages, comapnies with exhaust heat and own generator have started using VAM systems which are available locally. the VAMs run on gas, wood or exhaust heat. Hence, an established VAM market (around 5 Mio USD) is in place which is one of the main drivers with regards to SAC in India.

So far, a hand full of projects has been realised (around 10), mostly R&D and some private (non commercial/green) initiatives are in place, using concentrated solar collectors + VAM

• Solar Energy Center, TERI, IITs,…
• Thermax (3-effect VAM with COP of 1.7-1.8), Baskara Solar, Gadhia Solar,..
• Mamata - only project with evacuated tube collectors, since 2006, little maintenance needed.
• still early pilot phase
• So far no coherent strategic approach, lacking knowledge sharing
• R&D for solid and liquid dessicant systems

Economics

• Investment costs 3x higher than conventional systems
• High capital investment and long payback periods (> 7 years) highest drawback
• investors very hesitant regarding investments with payback >3years
• Most feasible for larger systems >100 kW
• Awareness raising needed
• Costs for chillers and solar components are decreasing
• Energy demand for control system and backup not to be underestaimated

Reliability of system and back-up storage

• as the main selling argument for SAC in India is the increased reliability an independence from the grid, a reliable system is key
• Improving quality of chillers and solar components, but still an issue
• Increasing reliability – storage/back-up necessary
• Solar as add on, not total replacement, otherwise very expensive
• Variation in irradiation and diffiulty of prediction - dynamic system needed
• Standardization of storage (cold and heat) necessary

System Design

• System design, integration and optimization prove difficult
• maintenance is a great issue
• Smaller VAMs under development, expensive
• Space availability for solar collectors and cooling towers is a challenge
• High humidity and water scarcity – find suitable technologies

Collectors

• Concentrating collectors (for higher temperature applications) face difficulties at the moment because of high fraction of diffuce radiation (dust, cloud coverage) in some regions
• Used as locally produced but also more expensive than flat plate/evacuated tube
• Achieve higher temperature lift, remove the need for wet cooling towers
• Maintenance issue
• Expensive, performance and tracking issues
• in some regions, non-concentrating collectors might be more suitable, also architectural integration possible, but lower temperatures, hence lower COPs
• Evacuated heat pipes not produced in India
• Efficiency of solar equipment needs to be improved

Recommendations for Pushing the Market Development for Solar Cooling i.a.

Training and awareness raising

• Work with associated associations (solar thermal, chiller manufacturers, society of engineers, architects, etc.) with regards to cooling
• establish a Technical working group on solar cooling with regular meetings
• Training programs for installers, planners of SAC systems
• Provide design tools for them on different levels
• Including SAC in engineering curricular
• Capacity development of architects in order to decrease necessary cooling loads and include solar cooling in their projects as a suitable solution
• broad awareness raising campaigns, lobbying

R&D

• Support visible and meaningful demonstration projects (with proven energy performance) to achieve standardization and guidelines, incl. showcases: collecting experiences, showing best practices as basis for awareness campaigns, potentials and limits
• Start keeping statistics on energy demand for cooling (split up industries)
• The market for room air conditioners is growing very rapidly in India. Hence, small SAC units which can mitigate the environmental impact of this trend are especially needed for the Indian market. Develop R&D with focus on small applications.
• Usability of residential solar water heaters for solar cooling (only 1-2 rooms needed)

Policy measures

• Roadmap for (solar/RE) cooling and
• inclusion of cooling into RE/solar targets at national/state level

Incentives Schemes

• Higher incentive in the early market status, reduced incentives when the market has started to take off: investment funds, tax reductions or credit programs with reduced interest rates. Based on standards of achieved energy/ CO₂ savings
• Review solar thermal subsidy scheme (often in place) with regards to cooling, adoptions might be necessary to make application viable – maybe special subsidy when connected to heating and hot water, if applicable
• Come up with easy financing mechanisms for solar /renewable cooling
• Phase out subsidies on conventional energies to decrease market distortion

Regulatory measures

• Inclusion of RE-cooling/SAC in building regulations (new buildings and refurbishment) – obligatory rating scheme for buildings
• Prohibition / discouragement of refrigerant with high global warming potential (GWP)

Further Information

• IEA Task 38 - Solar Air-Conditioning and Refrigeration -IEA Solar Cooling Position Paper
• Feasibility checklist for solar cooling projects (European focus)