Solar Cooling

Background

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.

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, 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 è lacking 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)

Mayor 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 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 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
• Maybe suitable alternative
• Different results depending on perspective and goal
• 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

The Ministry of New and Renewable Energe (MNRE) proposed an Action Plan on Solar Cooling in 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.

Further Information

IEA Task 38 - Solar Air-Conditioning and Refrigeration -IEA Solar Cooling Position Paper