Energy Efficient and Solar Cooling
Profile of Market Area
Already today many techniques are available which allow the use of solar thermal energy for air-conditioning of buildings. Our services in this technology area comprise consultance work for planners, carry-out of feasibility studies, system simulations on different levels of detail and accompanying pilot installations including a scientific monitoring phase. For many years only plants with medium and high cooling capacities (> 35 kW) for larger buildings were available. However, recently several companies started to produce thermally driven cooling systems also for the range of low cooling capacities (> 5 kW) and offers either prototypes or even standardised market products. Thus in the mid term it will be possible to realize solar cooling and air-conditioning also for small capacity applications.
Services and Expertise
Frequently Asked Questions
Which technologies are available
for solar air-conditioning today?
At first a general distinction between air handling units and water chillers has to be made.
The function of air handling units is to supply fresh air to a room or a building and thereby to replace exhaust air. At the same time latent loads (water vapour, released by persons or processes) are removed. In many cases the fresh air supplied to the building is cooled down and/or dehumidified before entering the room. Both these thermodynamic functions – lowering temperature and humidity of the fresh air – can be realized by an air handling unit in which sorptive dehumidification and evaporative cooling are combined. During sorptive dehumidification water vapour is being removed from the fresh air and bound by a sorptive material. The technique which is employed most frequently today uses so called sorption wheels. In thiese wheels a sorptive material is fixed in the matrix. The air passes the wheel trough chanels and thereby the humidity is transferred from the air to the sorptive material. Most rotors today use silica gel as sorptive material but also wheels employing a matrix out of lithiumchloride/cellulose are available. Due to its limited sorption capacity the sorptive material has to be regenerated after a certain time, i.e., the bound water molecules have to be removed. Here solar energy can be used: by means of solar thermal collectors – either using directly solar air collectors or solar water heating collectors in combination with an water-to-air heat exchanger – an air stream is heated up which flows through the sorptive material on the regeneration side of the wheel. Thereby the bound water is removed and exhausted to the ambient air.These open cooling cycles are also referred to as desiccant cooling systems or desiccant and evaporative cooling systems (DEC).
Systems based on chilled water use components installed in the room which are flown through by chilled water and thereby take up room loads. Examples are fan-coil units, chilled ceilings, chilled beams etc.. With some of these components, such as fan-coil units, it is also possible to dehumidify room air by cooling the air below the dew-point. Here a connection to the waste water system is necessary in order to lead away the condensate. Supply of chilled water can be realized using thermally driven chillers, which are operated using heat from solar thermal collectors or other heat sources (e.g. combined heat and power system). The most common technique is the absorption chiller in which a liquid sorptive material is used to run the cycle. In most cases of comfort air-conditioning a liquid lithium-bromide-solution is employed for this purpose. Another option is the use of solid sorptive materials such as silica gel or zeolite. In both cases hot water (or steam) is required for operation of the machines; for this purpose solar thermal collectors may be used.
Are systems available for residential
Commonly desiccant air handling units are available for nominal air volume flow rates higher than 3000 m3/h; this value is far to high for instance for a single-family house. For many years now water chiller with a nominal capacity below 35 kW was available on the market. But after several years of R&D, recently several companies start entering the market with water chillers in the range of small capacitites starting at about 5 kW. Both technologies, absorption and adsorption, are used for this purpose. Only very few systems are installed today but it can be expected that in the near future this will become a common technology.
Who supplies solar air conditioning
All systems which are in operation today have been individually designed. Almost no companies offer complete solar air conditioning systems. Today plants are usual with a nominal capacity of about 15 kW or 3000 m3/h, respectively. Typically these are applications in the commercial sector. In this respect normally planners are key players, who compose a system from appropriate components. New components which are recently introduced into the market allow the installation of pilot plants in the small capacity range.
What is the cost of solar air conditioning?
Is this technology economically viable?
Today thermally driven cooling and air-conditioning is usually applied if a „cheap“ heat source is available, such as waste heat, district heat or heat from co-generation. Heat from solar thermal systems leads to low operation costs while the investment costs are high due to the more components needed. Based on today’s energy prices and market prices for solar collectors solar air conditioning is not economically viable in most cases. Of course, the amount of extra costs depends on many boundary conditions and in particular the cost for electric peak consumption. Naturally the conditions are more favourable in sunny areas which also show a higher demand for summer air-conditioning.
How can Fraunhofer ISE help during
planning and installation of solar air conditioning systems?
In a special project we can start to analyze the demand for air conditioning, for instance by building simulation. Based on our expertise in building physics we are able to provide assistance in measures focussing on the reduction of cooling loads or the use of passive cooling technologies. Then we can help the planer in finding the best concept to install a solar air conditioning system. Based on annual simulation studies annual energy and cost balances can be computed. Often such a study can provide important information for decision making at comparatively low cost. During installation of a plant we can assist the planer and installer by giving hints on details of the hydraulic design and the system control.
Does Fraunhofer ISE develop new
concepts or systems?
Here, our key expertise is focused on solid sorption technology. In this area we have long year experience in the field of materials and their characterization as well as in development of processes and components. Our aim is to develop advanced systems with increased performance – in the field of open cycles as well as closed cycles – and with a particular focus on systems in the small capacity range. Promising new concepts exist, which are ready for development of commercial products in co-operation with partners from industry.