Depending on the purpose which solar heat is meant to be used for, different types of collectors can be chosen. In combination with a heat storage system, a significant share of the required heat can be provided CO2-free with solar energy, especially in the summer months.
Low Temperature Absorbers: Low temperature absorbers are the simplest type of collectors. These absorber mats made of specific organic materials (plastics, EPDM) are used to warm up the solar fluid. They can be efficiently used at temperature levels of up to ca. 40 °C. Therefore, they are suitable to warm up cold liquids to ambient temperature or as a heat source in combination with heat pumps.
Flat Plate Collectors:The metallic solar absorber of flat plate collectors is fit between a transparent cover and a heat isolation plate. This construction minimizes the collector’s heat loss and makes it possible to provide temperatures of up to 100 °C efficiently, depending on the collector’s design. The size of a single collector is normally between 2 m² for compact modules and up to 12 m² for large area collectors.
Vacuum Tube Collectors: By means of convection and heat conduction, heat loss can be reduced drastically with vacuum tube collectors. Hence, significantly higher yields are possible at higher temperatures. This type of collector works most effectively at a temperature range of 80 to 130° C, depending on whether or not it is equipped with a mirror behind the tubes (CPC-collector). Vacuum tube collectors can either work by direct flow heat conduction or by the Heat-Pipe-Principle.
Air Collectors:Air collectors work without a liquid heat-transfer-medium and are thus especially suitable for drying operations. Air collectors are available as tube collectors (double sided Sydney-Tubes) or flat plate collectors with open side fronts.
Concentrating Collectors:Using mirrors (like the Fresnel-Collector shown in the picture) or lenses to concentrate the solar radiation on an absorber, concentrating collectors are especially suitable for regions with high direct radiation. The mirrors’ orientation tracks the sun steadily to effectively reflect the radiation to the absorber. This collector design is particularly suitable for a temperature range of 150 to 400° C.
Usually a buffer tank is an integral part of a solar process heat plant, because the solar supply does not always correspond to the heat sink’s demand temporally. Next to phases with a low heat demand during a production day, there can be whole days with no heat demand e.g. at weekends. Since typically there is a very low heat demand, or even none at all, at industrial applications on the weekend, a buffer tank should be dimensioned in a way that it is able to store the solar yield of at least one day. For this purpose, usually water tanks which can be operated pressureless at up to 95° C or under pressure at up to 150° C are used.
Depending on collector area and the specific conditions of the heat sink, different volumes of storage are necessary for efficient system operation. It is recommended to provide the required volume with a single tank if possible. Besides ideal charge and discharge, improved temperature stratification, and low heat loss, the single tank solution is usually also cost effective. By now there are several manufacturers offering pre-configured, welded-on-site, or modular built tanks, for example made of steel, plastic, or GRP. These storage technologies make it possible to realize the necessary volume with a single tank, while taking into account the local conditions like ceiling height or the size of doors. Due to higher heat loss, greater effort of installation, and a raised potential for error, the use of multiple smaller tanks in series or in parallel is not recommended.
There are different possibilities to integrate the storage to the system: Typically, the solar circuit, which in most cases is operated with a water-glycol-mixture, is divided from the storage circuit by a heat exchanger. To charge the tank with solar heated water, it is usually designed with two inlets on different heights. Potentially the solar system can also be connected to the storage directly, without a heat exchanger. In this case, water flows through the collectors, which requires means of active frost protection.
Depending on the heat sink to be provided with solar heat, unloading of the tank can also be done directly or indirectly. In case of direct feed, the process medium to be heated (e.g. water for cleaning purposes) is directly led to the buffer tank, heated by the solar system, and led directly from the buffer tank to the consumer. Depending on the medium to be heated with solar energy (e.g. alkaline, milk, or drinking water), an extra heat exchanger may be necessary to separate the buffer tank from the heat sink.
Regarding the integration of a solar heat plant, there is a principal distinction between the integration on supply level and the integration on process level. Many companies in industry and commerce have a central boiler house for the production, with a connected heat distribution network. Depending on the flow temperature, the heat is distributed by steam (140…200°C), hot water (90…160°C) or warm water (<100°C) and is fed to the heat sink directly or indirectly by a heat exchanger.
In order to determine at which point of the system the integration of solar heat is most reasonable, especially in bigger companies, the existing system’s heat sinks should be evaluated and compared. The three main factors to compare are the temperature, the load profile and the expense of effort to integrate solar heat into the existing system.
Temperature:With increasing average collector temperature, the collector’s efficiency decreases; this in turn also leads to a reduced yield. So, in Germany and other countries with similar climatic conditions, the temperature a solar system has to provide is the most crucial factor for the decision about the point of integration. Therefore, applications with low process temperature or those were pre-heating is possible are preferable options for a solar assisted heat supply.
Load Profile: Regarding the load profile, processes with a long and constant weekly and yearly heat demand are to prefer. Since most companies only produce from Monday to Friday, or sometimes additionally on Saturdays, the buffer tank of a solar system should have enough volume to store at least the solar energy yield of one summer day. This way, the buffer tank’s capacity is sufficient to compensate possible variations of the load profile during a day. This way, the buffer tank’s capacity is sufficient to compensate possible variations of the load profile during a day.
Integration Effort:The effort to integrate solar heat may vary dramatically depending on the existing system’s technology. While for example the warm water supply for cleaning purposes may only require a heat exchanger plus periphery (pump, valve, pipes, etc.) for integration, the heating of baths or machines can be way more complicated. While for example the warm water supply for cleaning purposes may only require a heat exchanger plus periphery (pump, valve, pipes, etc.) for integration, the heating of baths or machines can be way more complicated.
In general, the integration at heat sinks operated by external heat exchangers are in most cases easier to realize.