06 August 2012
We tend to think of solar energy as a source of electricity or heat but solar power is increasingly being used in cooling systems. Such systems are attractive because cooling is most needed in areas with the greatest solar potential. Using photovoltaic (PV) solar panels to generate electrical power for a conventional air conditioning (AC) system or refrigeration units is one solution, although the high power requirements of these makes this approach difficult.
However, there are several other, more economically and technologically feasible methods, including sorption (adsorption and absorption) chillers, desiccant-based evaporative cooling systems and hybrids, which combine elements of different technologies.
Hybrid solar AC
For solar cooling to reach its market expectations, more cost-effective products and novel technologies must be developed.
An alternative to the PV-AC approach is the hybrid solar air conditioning system. This uses similar equipment as a conventional AC system but with a specialised solar collector located between the compressor and the condensing coils. This collector is a highly efficient vacuum tube filled with an organic liquid that heats the liquid to over 350°C. This superheats the refrigerant to above the temperature that the compressor could achieve by electrical means, allowing the gas to change back into a liquid much more quickly and dramatically reduces the energy requirement of the compressor. The gas condenses back into saturated gas in the first third of the condensing coil rather than the final third. Therefore, by the time the refrigerant reaches the expansion device in the inside coil, it is already almost a liquid. This causes the near-liquid refrigerant to be more efficient at absorbing heat, making it 5-6 degrees cooler in the inside coil, thus delivering colder and drier air.
The resulting efficiency derived from the solar collector allows the refrigerant to work more effectively and systems do not require any additional components. This technology is not yet widely used but is starting to make an impact in the US. For example, in 2011, Fafco Solar Energy installed a system at the Cape Coral Eye Center in North Fort Myers, Florida.
Absorption cooling systems operate on a totally different principle. The thermodynamic cycle involves three phases: evaporation, where a liquid refrigerant evaporates in a low partial pressure environment, thus extracting heat from its surroundings (the refrigerator); absorption, where the gaseous refrigerant is absorbed (dissolved into another liquid) reducing its partial pressure in the evaporator and allowing more liquid to evaporate; and regeneration, where the refrigerant-laden liquid is heated, causing the refrigerant to evaporate out. It is then condensed through a heat exchanger to replenish the refrigerant supply in the evaporator.
Solar-powered absorption chillers rely on the heat generated from tube collectors. Efficient absorption chillers generally require water with a temperature of at least 190°F (88°C) but common, inexpensive flat-plate solar thermal collectors can only heat water to about 160°F (71°C), so high-temperature, flat-plate, concentrating collectors or evacuated tube collectors are needed to produce the higher temperatures. As a rule of thumb, around 3 square metres of absorber are needed for each kW of cooling capacity. Although earlier systems used ammonia, most of today’s use water as the refrigerant and lithium bromide as the sorbent, as this combination yields an improved coefficient of performance (COP).
Several very large systems have recently been constructed and an example is at the headquarters of Caixa Geral de Depósitos, Portugal’s largest bank, in Lisbon. This building has 17 floors, 100,000 square metres of office space and houses 6000 employees. It is equipped with 1579 square metres of solar collectors and generates 545 kW of cooling power. The system was produced by SOLID Gesellschaft für Solarinstallation und Design GmbH and yields electricity savings of 1,252 kW per year and reduces the building’s CO2 emissions by 408 tons per year.
Solar collectors on the roof of the UWC-SEA building. Credit: SOLID Gesellschaft für Solarinstallation und Design GmbH, www.solid.at.
An even larger system, also from SOLID, is at the UWC-SEA (United World College South East Asia) campus in Singapore. This is believed to be the world’s largest solar-powered cooling plant and uses solar thermal collectors with a total area of 3900 square metres (see photo above), which power a lithium bromide absorption chiller with a cooling capacity of 1,575 kW. Further, the solar system can supply up to 100% of the building’s hot water demand. Adsorption systems are under development whereby the liquids are replaced by solid sorbents such as silica gel or zeolite materials. These can be powered by solar thermal vacuum tubes or flat-plate collectors.
In evaporative cooling systems, water’s large latent heat of vaporisation is exploited. The temperature of dry air can be dropped significantly through the phase transition of liquid water to vapour, a process that requires much less energy than conventional refrigeration. Air is moved by a centrifugal fan or blower and a water pump is used to wet evaporative cooling pads. The fan draws ambient air through vents on the unit’s sides and through the damp pads. Heat in the air evaporates water from the pads that are constantly re-dampened to continue the cooling process. Then, cooled, moist air is delivered into the building via a vent. This is a long-established cooling technique but solar-powered versions are a far more recent innovation. In these, flat-plate and vacuum tube collectors are generally used and hybrid systems are becoming available. These offer the option of solar or mains electrical power; the latter being used when insufficient sunlight is available.
Realising the potential
According to a report by the International Energy Agency (IEA), by 2050 solar technologies could provide approximately 17% of the total energy used for cooling world-wide. Many installations have been realised and the technology has shown that significant energy savings and a reduction in CO2 emissions are possible. However, it is fair to say that solar cooling is at a critical stage, having reached the level of early market deployment. But for the IEA figure to be realised, more cost-effective products and novel technologies must be developed to stimulate the market.
Written by Rob Bogue, Contributing Editor, UK, Solar Novus Today