Components of active thermal solar systems

Types of collectors

Active thermal solar systems are systems in which the entered solar radiation is converted using technical equipment (collector) for heating purposes and delivered to a consumer (hot water storage tank, space heating, swimming pool). The central part of a thermal solar system is the collector. The four typically collector constructions are:

• Plastic absorber
• Flat plate collector
• Evacuated collector
• Air collectors

Plastic absorber for heating of a swimming pool

Due to their limited pressure and temperature durability, plastic absorbers are mainly used for the heating pool water. In this case, the desired temperature level is only a few degrees higher than the ambient temperature. Thus, simple plastic absorbers can usually be mounted uncovered on a flat roof or on a lawn. Since they are made entirely of plastic, they have the advantage of a single-circuit operation. The chlorinated pool water is directly pumped through the absorbers by a circulation pump and no heat exchanger is needed.

Figure 4: Plastic absorber for swimming pool heating

Flat plate collectors

The flat plate collector consists essentially of the collector box, the absorber, the heat insulation and the transparent cover.

Figure 5: Basic layout of a flat collector (assembly in roof)

In the first instance, solar radiation hits the transparent cover of the collector. Due to reflections both on the surface and at the interface (transmission) of the cover, some of the radiation is lost for further utilisation in the collector. Depending on the type of covering, the solar radiation which strikes the absorber is almost entirely converted to heat. The coating should have a high absorptance and the lowest possible emittance. The absorption capability is characterised by the absorption coefficient * and it is mainly determined by the black colour of the absorber. The absorption coefficient for a solar coating with a solar varnish as well as for good selective covering is between 0,94 and 0,97. The emission coefficient lies between 0,86 and 0,88 for coatings with solar varnish; for selective covering it is only 0,05 to 0,20.

The covering can be applied either by spraying (in the case of a coating with a solar varnish), by galvanic means or by means of an adhesive film (in the case of selective covering). Good selective coatings are offered since 1996 where the special physical process of sputtering has been used. Compared to galvanic methods, this technique results in a much more ecologically benign covering which also requires less energy.

Heat losses are also caused as a result of convection in the collector and occur at the back side of the absorber.

Figure 6: Losses of a flat plate collector

Evacuated collectors

For technical reasons, most of the evacuated collectors are constructed in the form of tube collectors. A thin absorber strip with selective coating is closed inside an heat-resistant glass tube of great light transmitting capacity. As a result of evacuation the collector has very little convection- and heat losses.

Figure 7: Vacuum-tube collector

Evacuated tube collectors obtain higher yearly solar yields per square unit in domestic hot water systems than flat plate collectors. However, as a result of their higher price, market penetration is still low. In Austria, the share of the market is about 1%.

Since the excess yield from vacuum tube collectors increases quite considerably, particularly in the high collector temperature range, they are ideally suited for the production of process heat.

Air collectors

The basic layout of air collectors is the same as for flat plate collectors. They comprise the collector box, the transparent cover, an absorber and thermal insulation on the back side. When it comes to the selection of materials the same basic rules are to be observed with regard to the components and weather-resistance as for a flat plate collector. Concerning the air collectors there are basically three construction types: collectors with a downstream, upstream or wetted absorber. Wherever higher air temperatures have to be reached, constructions with back-wetted absorbers would be advantageous since the heated air is not directly in contact with the cold upper cover.

Figure 8: Principal layout of a solar air collector with a back-wetted absorber

Applications for collectors

"Contrary to the promises made by the manufacturers, there is not a best collector but rather one or several suitable products for each application" /7/. A Selection of the suitable collector for the respective application is shown in figure 9.

Characteristic values of flat plate collectors

The collector efficiency curve is an important physical property of a solar collector. The efficiency of a collector is defined as the ratio of the energy amount transferred from the collector to the heat transfer medium to the incident radiant energy on the collector. Especially for temperatures (heat transfer fluid) higher than 40ºC, high efficiency values are desirable for flat plate and evacuated - tube collectors. The efficiency depends on the quality of the absorber surface, the geometry of the absorber, the heat conductivity of the absorber material, the transparency of the cover and the heat losses of the collector through infrared radiation, conduction, and convection. A quantitative comparison indicates that the efficiency is particularly dominated by the radiation losses. The efficiency for a certain collector is not a fixed value, but rather it is dependent on the application, e.g. temperature levels, wind speed, etc.

Figure 10: Collector efficiency curves for various types of collectors

Conversion factor [eta]0

The conversion factor [eta]0 is defined as the maximum efficiency of a collector under the precondition that the average temperature of the heat transfer medium in the absorber equals the ambient temperature.

Heat loss coefficient k

The heat loss coefficient is the average heat loss of a collector per m² effective collector area divided by the temperature difference between the absorber and the ambient temperature.

The k-value of the collector is described by the two values k1 in accordance with the share dependent on the temperature and k2, the share that is not dependent on the temperature. The conversion factor [eta]0 of a collector should, therefore, be as high as possible and the k-value as low as possible. The collector parameters are ascertained using a standardised testing procedure conducted by an authorised test institute.

Technologies for storage tanks

After the collectors, the hot water tank is the second most essential component in a thermal solar system. The correct choice and dimensioning contributes decisively to the solar fraction achieved. The most important hot water tanks used for preparation domestic hot water and for space heating will be described now:

Hot water storage tank

The most common building form is an upright cylindrical steel storage tank (hot water tank), which has to fulfil certain requirements according to what it is used for. Because of the constant inflow of cold water, that is also enriched with oxygen, the interior of the storage tank has to be coated with a non-poisonous coating as a protection against corrosion. The types of coatings range from very temperature-resistant enamel layers to the cheaper storage tanks with a plastic coating, which however can only be used for lower temperature ranges. Steel storage tanks often have an additional corrosion protection because of possible defects in the interior coating of the storage tank. The cathodic corrosion protection, consisting of a reactive anode or a external current anode, prevents copper ions from the pipeline or from the ribbed heat exchanger from being deposited on these defects and leading to contact corrosion there.

Figure 11: Example of a solar storage tank: solar register storage tank (Austria Email)

Stainless steel storage tanks are more seldom but quite acceptable. Their distinguishing characteristics are a high corrosion resistance and durability.

Plastic storage tanks are not wide-spread. There are, however, some interesting new developments in the form of unpressurized operated heating energy storage.

The loading of energy in the domestic hot water storage tank is usually carried out via a pipe register (bare-tube heat exchanger) already permanently built-in, or via ribbed pipe heat exchanges, that can be subsequently built-in if necessary using flange plates.

Domestic hot water tank in space heating tank

The tank in tank represents a solution which is striking because of its very simplicity which is to include the domestic hot water storage tank in the energy storage tank. A boiler is welded onto an energy storage tank made of steel. The volume of the boiler is kept relatively small for reasons of hygiene, however, it suffices to prepare the warm water as a result of the combination of the storage tank and flow heater principle. The cold water inlet is modelled in the lower storage tank area as the preheating section in the form of a bare-tube heat exchanger. The advantage of this system is its hydraulic simplicity.

Figure 12: Domestic hot water tank in space heating tank, energy storage tank with an integrated precious steel boiler (Feuron AG)

Energy storage tank for solar space heating

The size of the storage tank is determined on the one hand by the solar fraction desired and on the other hand by the additional heating system. Since only heating water with a low level of oxygen circulates in storage tanks of this kind, no special arrangements are needed with regard to corrosion. The storage tanks are only treated with a passivating coating on the outside. In a single family house it makes economic sense to use energy storage tanks with a size of 1 to 5 m³. To reduce the blending losses when loading the energy storage tank, we recommend that the energy storage tank should be designed with layer loading equipments. The operating principle for layer loading can be explained as follows:

As long as the water rising in the layer loader has a higher temperature than the water in the surrounding storage tank layer, the flaps remain closed due to the hydrostatic difference in pressure between the storage water and the water inside of the loader pipe. In the case that the temperature and the density are equal within and outside the pipe, the pressure is raised on the respective flap and the rising water can emerge through the flap. Another possibility for layer loading is offered by valve control. This system comprises valves and several pipe connections (at different heights) to the energy storage tank. Depending on the water temperature (solar preheating temperature and water temperature in the energy storage tank) the layering of the water into the energy storage tank is performed by regulating the valves.

Figure 13: Energy storage tank with the SOLVIS layer charging system (PINK)

Control Systems

A very important component in a thermal solar plant is the electronic control of the temperature difference: by the means of two temperature sensors, the absorber temperature and the storage tank temperature near the heat exchanger are compared with each other and the circulation pump is switched on when the absorber temperature is a defined value higher than the temperature in the storage tank. If these circumstances do not longer apply, the pump is switched off by the control system.

The regulating systems offered on the market by different manufacturers are designed in such a way that an electronic control system is also available for each hydraulic scheme. Furthermore, there is a strong tendency towards individually programmable control systems with which basically all the hydraulic standard versions and a number of alternatives are possible. In the same way different concepts for the preparation of domestic hot water are possible in combination with solar space heating.

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Sources

/7/ U. Frei, Solarenergie Prüf- und Forschungsstelle, Rappersvill, CH Seite 83