Solar cooling – while the concept sounds contradictory – actually represents a perfect correlation between solar supply and cooling demand. When the sun is shining and temperatures are rising, that’s when we need cooling the most. Yet, despite this natural connection, solar cooling is still a relatively nascent market, with just under 1000 installations worldwide, according to the International Energy Agency in 2011.
Excessive air conditioning use is one of the most common causes of strain on the grid, leading to rolling blackouts from southern California to New Delhi. Displacing cooling powered by electricity, or other traditional energy sources such as natural gas and propane, solar thermal cooling reduces customers’ utility bills and greenhouse gas emissions, while serving as a hedge against grid constraints.
The largest solar cooling installation in 2011 was a cosmetic factory in Greece with two 100 tonne adsorption chillers. Today, the largest system is more than double this size. A 425 tonne installation at the United World College in Singapore uses 3900 m² of solar collectors to provide heating and cooling for the campus.
Demand for air conditioning is expected to more than double worldwide by 2050, driven by a growing population and rising income levels in the developing world. And with energy and infrastructure resources limited, the market for solar cooling is bound to heat up.
Solar cogeneration technology combines photovoltaic (PV) and solar thermal technologies into a single system, producing electricity, heating and cooling solutions. Traditional PV panels absorb only a small fraction of the sun’s energy, typically 15%. Solar cogeneration, however, increases the overall system’s efficiency to 75%. The ability of solar cogeneration to produce high temperatures and electricity in a single system provides a flexible platform for solar cooling with the highest efficiency.
Solar Cooling Site Integration
While solar cogeneration can integrate with both thermal chillers and desiccant coolers, adsorption and single-effect absorption chillers are ideal for simple and safe integration with solar and existing chilled water systems.
Almost all solar cooling installations out there today are absorption chillers – which were the first thermal chillers commercially available and have been the standard install choice ever since. In typical site integrations, a thermal chiller sized to meet a fraction of customer cooling demand ties into the existing chilled water system, displacing traditional electrically driven cooling. Sizing for a fraction of the load (instead of the full load) allows the chilled water produced by solar to flow through the pipes and air-handling units on-site without upgrades or modification.
Southern California Gas Company (SoCalGas) is the first commercial entity to deploy solar cogeneration cooling. Partnering with Cogenra Solar, the utility installed a 20-module solar cogeneration system on the roof of its Energy Resource Center (ERC) outside Los Angeles in May 2012. Instead of using electricity to run mechanical chillers, solar cogeneration provides power to one of the ERC’s absorption chillers and supports the building’s air conditioning system. The 50.2 kW installation produces electricity and heat to drive a 10 tonne single-effect absorption chiller. A high efficiency boiler is connected in parallel with the system for use when the sun is not shining.
In addition to financial benefits from energy savings, local and international governments are offering substantial incentives for solar cooling systems. For example, in the US both Tucson Electric Power (TEP) and Arizona Public Service Company (APS) in Phoenix, Arizona offer incentives specifically for solar cooling. APS offers an up-front incentive up to $75,000, and both APS and TEP offer 10, 15, and 20 year production-based incentives with the 20 year production based incentive of $0.080/kWh of heat produced for cooling.
Traditional air conditioning typically accounts for more than half of a building’s electrical usage, and cooling consumed 13% of total world buildings’ energy use in 2009. The number and size of solar cooling installations is expected to continue to increase as demand grows for air conditioning and the cost of electricity rises. Addressing this growing demand with efficient solar cogeneration that both displaces and produces electricity could be an optimal solution.
Andrea Gains-Germain is a product manager, and Mani Thothadri is a senior director in charge of product management and marketing, at Cogenra Solar.