Solar-Thermal Cooling: Rio de Janeiro

Solar thermal cooling can improve the energy efficiency of buildings

The Santa Clara University solar decathlon house is a demonstration project fitted with a solar thermal powered cooling system. Photo by congressman honda, licensed under creative commons.

The use of solar thermal energy to drive cooling systems is becoming more common, though commercial systems are not yet widespread.

The International Energy Agency (IEA) of the Organization for Economic Cooperation and Development (OECD) has several projects that involve using solar energy to provide building cooling.

This technology is usually known as solar cooling (in Portuguese climatização solar ou ainda refrigeração solar).

Solar cooling has the potential to be usefully applied in commercial and office buildings, where the demand for air-conditioning is correlated with the greatest availability of solar radiation.

In most parts of the world a large proportion of electricity is produced by the combustion of fossil fuels. In this context, solar cooling technology has the potential to lower greenhouse gas (GHG) emissions by reducing the electrical demand for air-conditioning. This may not be the case in Rio de Janeiro, where the electricity supply originates primarily from renewable sources. This means that the use of solar thermal cooling in Rio may not appreciably reduce GHG emissions relative to standard electrically powered air-conditioning. However, opportunities for renewable power generation are themselves finite and most have other environmental or social impacts.  Further, conventional air-conditioning is responsible for very large spikes in power demand which might need to be met using fossil fuel power stations and certainly adds considerably to infrastructure costs (generation and distribution).

Large scale thermally driven cooling is already available – but small scale technology is still emerging [1].

What is Solar Cooling?

Most solar-thermal cooling systems use solar-thermal collectors just like those used for solar-thermal domestic hot water systems, but instead of using the heat for hot water purposes, the heat is directed at an absorption chiller.

An absorption chiller is thermodynamically very similar (in principle) to a conventional (vapour compression) style chiller, but where motive power is provided by heat rather than by an electrically-powered compressor.

Absorption chillers are much less efficient than conventional chillers, with between 0.65 to 1.2 kW of cooling provided for each 1.0 kW of heat input. This compares with about 3.0 kW of cooling per kW electricity input for a conventional chiller. The case for absorption chillers relies upon the energy input being relatively cheap and/or low carbon (i.e. the Sun).

The key technical disadvantage with absorption chillers is that they require quite high temperatures to drive them, typically upwards of about 90° C (the higher the temperature, the higher the efficiency). If the temperature drops below the critical minimum, the cooling effect will stop completely. For a solar-thermal system, it can be very difficult to maintain such high-grade heat.

Solar Thermal Desiccant Cooling

An alternative type of solar-thermal cooling uses desiccants. Most commonly these would be solid desiccants arranged in a wheel that rotates between two air streams.

One air stream is warm and/or humid air drawn from outside. As this air passes through the desiccant wheel, moisture is absorbed. As the moisture is absorbed, the air temperature rises (the more familiar process is air cooling when water evaporates – so the reverse must occur when water is removed). At this stage the air is dry but very hot – maybe 70° C or so. Air at 70° C is no good for keeping people cool, so it is passed through a heat exchanger to get rid of its surplus heat. At 70° C the air is so hot that it can easily lose heat to anything around it – usually the cool air that is displaced from the building by the fresh air being supplied.

At this stage, the air is about the same temperature as outside ambient air but much drier. The solar-thermal heat is now used to regenerate the desiccant (purge the desiccant wheel of moisture) to allow the process to repeat.

In their simplest form, desiccant systems just provide dehumidification – which is useful enough in humid environments – but it is possible to take the process further. If the air is over-dried, excess heat rejected, then evaporatively re-humidified, an overall reduction in both temperature and humidity can be achieved. In this way desiccant systems can provide full air-conditioning.

This technology is counter-intuitive because heat and water are supplied in order to cool and dehumidify. The main advantage is that the desiccants will regenerate at cooler temperatures than those required by absorption chillers, making them more suitable for solar-thermal systems.

There is also a liquid desiccant variant using, in effect, salt-water (lithium chloride or lithium bromide). Desiccant systems are much simpler to maintain than absorption chillers and do not require such specialist know-how.

For a more complete description of solar thermal cooling techniques, see this resource provided by the European Solar Thermal Industry Association.

Solar Thermal Cooling System Components

There are a number of chillers available in the Brazilian market, and wet-cooling towers are also available (e.g. International Refrigeracao). It should be noted that not all solar thermal collectors are suitable to drive solar thermal cooling systems, so care should be taken to match compatible system components.

The Economics of Solar Thermal

Regular air-conditioning units (for example, split air-con units) are produced in bulk, and are far cheaper to install than solar-thermal systems. Solar cooling systems are substantially more complex than standard air-conditioning, and require careful consideration at the design stage.

One demonstration project at Guaratingueta reported installation costs for a solar thermal cooling system that was almost 12 times more expensive than standard split air conditioning. Installation costs for a solar-thermal system amounted to R$ 5461 per kW cooling capacity, compared with R$ 456 per kW for split air-conditioning.

Running costs for solar-thermal systems may be up to five times cheaper than standard air-conditioning – although maintenance costs for solar-thermal cooling installations are likely to be higher than conventional HVAC. Taken together, it is likely that solar thermal cooling systems are not economically feasible unless finance is available at less than 1.5% annual interest, and electricity prices exceed 0,60 R$/kWh. If these conditions are not met, the installation is likely to have a payback period in excess of 21 years – no “income” during lifespan [2].

Even demonstration projects for solar thermal cooling often require backup with standard air-conditioners, or an alternative heat source. In the context of Rio de Janeiro, the most prudent course of action might be to employ standard but highly energy efficient HVAC technologies, combined with passive energy-efficient design measures.

References

  1. International Energy Agency, “Technology Roadmap: Solar Heating and Cooling,” 2012. [Online]. Available: http://www.iea.org/publications/freepublications/publication/2012_SolarHeatingCooling_Roadmap_FINAL_WEB.pdf. [Accessed: 30-Oct-2012].
  2. F. R. Till, “Technical and Economic Assessment of Medium Sized Solar-Assisted Air-Conditioning in Brazil”  Dissertação (Mestrado em Engenharia Urbana e Ambiental) – Pontifícia Universidade Católica do Rio de Janeiro, Rio de Janeiro, 2010.