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Solar-thermal cooling of chill houses in Umkirch near Freiburg.
© Kramer GmbH
Concepts and technologies for air conditioning buildings
Themeninfo III/2016
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Cooling with solar heat

The cooling or air conditioning of buildings with solar heat has a particular appeal because the heat demand and supply are usually consistent with each another. Cold stores in southern climates and many process refrigeration systems also require considerable energy when the sun shines intensely. The use of solar cooling systems instead of electric chillers also eases the grid, particularly at peak load times. Solar thermal cooling could develop sales markets, for example, in the Mediterranean region but also play a small part in Germany's "Energiewende" – its energy transition. This is reflected in the research funding provided by the German federal government.

In technological terms, German research institutions and SMEs have established a leading international position thanks to their intensive research. However, this young industry sector is facing double competition: on the one hand, well-known companies from the cooling sector, particularly from Asia, are surging onto the international markets; on the other hand, new competition is growing with regard to the technology, since the significant decrease in the cost of photovoltaics has also made solar electric systems with compression chillers increasingly attractive.

Most scientists, however, see scope for both technologies. In Europe alone, the need for cooling will quadruple between 1990 and 2020 according to a report for the European Commission. An advantage of solar thermal systems is that they can be flexibly combined with other heat sources. For example, industrial waste heat or energy from cogeneration could also be used. Compared with conventional cooling technology, solar cooling requires high initial investment but has subsequently lower operating costs. It is therefore particularly suitable when the cooling load, solar radiation and price of electricity at the location are high. More research needs to be conducted on the components as well as on the system technology. The following sections provide an overview of the various open and closed methods and provide an insight into the research.

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Solar heat replaces grid power

Even in temperate climates, numerous buildings have to be air conditioned. In conference centres, theatres, department stores or high-rise buildings, only indoor air handling units are usually capable of ensuring a comfortable indoor climate. Solar-based methods, however, can particularly lower the electricity needs at peak load times.

Many countries in sunny regions suffer from high loads on the electricity grid for handling cooling and air conditioning tasks. In some Mediterranean countries, more than half of the total electricity produced is used for air conditioning buildings in summer. Even a significant increase in building standards would not change anything in the short term. The use of solar energy for cooling and air conditioning would seem obvious here, as there is a high correlation between sunlight, ambient heat and the cooling requirement. Solar cooling can effectively reduce the electrical energy consumption for cooling and air conditioning and thereby counter the growing burden on power grids in sunny countries.

Less clear, however, is the situation for central European climates. Less than 5 per cent of the total electricity produced is used in Germany for air conditioning buildings. However, there is a significantly greater cooling requirement as part of food production and storage, as well as in industrial refrigeration. In these areas the cooling requirement is not temperature- and irradiation-dependent to the same extent. Nevertheless, a growing demand for comfort air conditioning can also be expected in Germany, even if the number of full load hours per year will be low in many applications.

Solar cooling and air conditioning refers to a process in which solar energy directly supplies a cooling or air conditioning process with energy. There is therefore a direct correlation between solar energy and the cooling process.

There are three basic approaches for providing cooling with solar energy:

  • The photovoltaic generation of electricity and its subsequent use in compression chillers
  • Thermo-mechanical systems (Vuilleumier and Rankine processes)
  • Solar thermal systems (desiccant and evaporative cooling (DEC)), ab- and adsorption cooling processes, steam jet cooling process).

This Themeninfo brochure focuses on solar thermal systems. Here it is differentiated between closed and open methods. Closed methods use ab- or adsorption chillers to provide chilled water that is used, for example, in chilled ceilings. Open sorption methods, on the other hand, condition the supply air. Here they not only reduce the temperature but also ensure a pleasant indoor air humidity.

In both cases, the system technology and collector system must be matched in terms of the size, suitability and control of the components. Ideally, the solar heat is used for other tasks, such as for domestic hot water or auxiliary space heating. Solar thermally driven cooling and air conditioning technologies have the following advantages:

  • They relieve the power grid, since they use little electricity.
  • The refrigerant (e.g. water) does not have global warming potential.
  • They are mostly operated at temperatures below 100 °C and are therefore suitable for stationary collector technology.
  • They can be combined with waste heat recovery.
  • They do not produce noise and vibrations.

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Addresses

Projects EVASOLK, Solarthermie 2000plus, ECOS, AgroKühl
Fraunhofer ISE

Service

BINE-Themeninfo III/2016
(PDF, 24 Seiten, 2,8 MB)

Authors

Dr Alexander Morgenstern
Edo Wiemken
Fraunhofer ISE

Dr Mathias Safarik
ILK Dresden

Peter Zachmeier
ZAE Bayern

Links

IEA SHC Task 48
Quality Assurance and Support Measures for Solar Cooling Systems

Green Chiller
Verband für Sorptionskälte e.V.