Are solar panels energy efficient

Solar heat pump system

Lexicon> Letter S> Solar heat pump system

Definition: a system for generating heat with a combination of solar panels and a heat pump

English: solar / heat pump system

Categories: renewable energy, basic terms, building services, heating and cooling

Author: Dr. Rüdiger Paschotta

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Original creation: December 25, 2013; last change: 03/14/2020


Pure solar thermal systems with solar collectors can generate heat for hot water preparation and heating support with very little primary energy consumption; only little electrical energy is required for a circulation pump and an electronic control. However, solar energy is often not available, or not available in sufficient quantities, when the greatest amount of heat is needed.

On the other hand, heat pumps, especially if they have a sufficiently good heat source such as B. can use the ground, reliably deliver heat (→Heat pump heating). However, they require significant amounts of drive energy, often in the form of electrical energy (→Electric heat pump).

In order to enable a reliable heat supply with lower overall primary energy consumption, solar thermal energy and heat pumps can also be combined into one Solar heat pump system - in quite different ways, which are described below.

Solar heat pump systems do not allow solar heating in the actual sense, in which the majority of the primary energy is replaced by solar energy. After all, the primary energy consumption of some (not all) solar heat pump systems is lower than that of pure heat pump heating.

Types of solar heat pump systems

Parallel systems

In a parallel system (Figure 1), the heat demand is met as much as possible directly with solar collectors, and a heat pump with a separate heat source generates additional heat as required. The collectors are mostly flat-plate collectors, sometimes also vacuum tube collectors. As a rule, the heat is first put into a shared buffer storage tank (solar storage tank) in order to achieve a certain decoupling of supply and demand. A stratified charge storage tank is ideally used. Only the top part of it is kept at a temperature that is sufficiently high for use, if necessary with the use of the heat pump. The rest of the storage is only charged with the help of solar energy, as far as it is available.

Parallel systems usually only achieve a small solar proportion of the annual heat production.

Unfortunately, solar collectors and heat pumps do not complement each other ideally here. Both work most efficiently at the lowest possible storage tank temperatures. The heat pump becomes less efficient if the collectors have already supplied heat, and vice versa the same applies. After all, a lower use of primary energy is likely to be necessary than with a pure heat pump system - admittedly with correspondingly higher investment costs. The share of solar heat in the annual mean is unfortunately quite limited in such systems. You will not want to dimension the collectors so large that there is a large excess of heat in summer. In winter, the contribution from the collectors is then relatively small.

Incidentally, it would make more sense to equip one house only with solar collectors and another with only a heat pump, rather than one house with both (all parallel system) and the other with conventional heating.

Serial system

A simple serial system is obtained (Figure 2) by using the solar collectors as the heat source for the heat pump - in the simplest case as its only heat source. This is usually set up in such a way that the collectors can load the storage tank directly when there is good solar radiation, i.e. without the heat pump having to run. In the case of weak solar radiation, the achievable collector temperature is not sufficient for this; then the heat pump brings the generated heat to a higher temperature level. Under certain circumstances, the collectors are barely any warmer than the surroundings, or even a little colder. You lose much less heat to the outside, or even gain some ambient heat, and accordingly provide more useful heat (especially in the transition period, sometimes also in winter). For this, however, the heat pump needs drive energy. The solar heat then replaces another low-temperature heat source for the heat pump instead of directly supplying useful heat. In other words, only anergy is gained with solar energy, not exergy.

This concept can also be built as simply as possible Lowest temperature solar panels that have poor thermal insulation or no thermal insulation at all. This can save costs and even increase the heat yield in operation with a heat pump, because z. B. Losses due to reflection of solar radiation on an insulating cover glass can be avoided. In this case, however, the heat pump is almost always required, as the higher temperatures are difficult to achieve for direct charging of the storage tank.

At night or in cloudy weather, the heating output is relatively low, even if the collector temperature drops slightly below the ambient temperature. If possible, the heat will be drawn from the storage tank and only recharged when the situation improves. Ideally, with the help of the weather forecast, the control would “know” whether it is worth waiting or the conditions are becoming even more unfavorable.

The low temperature operation requires that the possible problem of condensation be taken into account.

At collector temperatures below the ambient temperature, condensation of air humidity can occur, possibly even the formation of an ice layer. Low-temperature collectors can be designed in such a way that condensation has as little adverse effect as possible, and even a temporary layer of ice is tolerated. Otherwise, the output of the heat pump can be reduced before the collector temperature drops too much - but of course the heating output collapses. This only works as long as the required useful heat can be obtained from the storage tank.

Since the serial systems described above can only provide limited heating power at times, they are usually not suitable for the complete heat supply of a house. You therefore have to add additional components such as B. a boiler can be added, which unfortunately drives up the investment costs further. However, a natural gas boiler can be implemented relatively inexpensively if a gas connection is available, and since it is only required for cold, cloudy days, the annual fuel requirement can be quite low.

Some serial systems use an additional low-temperature heat store (Figure 3), often in the form of an ice store, instead of a boiler. Since the latent heat (solidification heat) can also be used here and thermal insulation of the storage tank is not necessary, a fairly high storage capacity is possible (more than with the above-mentioned hot water buffer storage tank), which is even sufficient for seasonal storage. So a lot of solar heat can be stored in warm weeks and used in cold weeks. (Unusable solar heat is unlikely to ever arise.) An additional boiler is not necessary if the low-temperature storage tank is large enough. The hot water storage tank can also be significantly smaller, as it is usually only required for hot water preparation, but not for heating.

Because the collector temperatures are usually very low, the use of corresponding low-temperature collectors makes sense, especially in systems with a low-temperature storage tank. There are also special collectors that use the outside air when the outside temperature is high enough.

The energy efficiency of such a system should generally correspond roughly to that of a pure geothermal probe heat pump heating system, since the temperature of the heat source is roughly the same in both cases. Since geothermal heating is usually more cost-effective, it should be preferred - provided that geothermal probes can be installed at the respective location. Otherwise, the ice storage solution can be quite attractive. In both cases, there is also the additional possibility of cold delivery z. B. for summer air conditioning. This additional use of the ice store or the geothermal probe can even reduce the energy requirement in later heating operation.

Combined systems

In a combined system (Figure 4), the heat pump can use both the collectors and another heat source such as B. use a geothermal probe. The control strategy can then be as follows:

  • When there is good solar radiation, the solar collectors load the heat accumulator directly.
  • If the solar radiation is too low, heat is extracted from the collectors with the help of the heat pump and fed to the storage tank - but only as long as the collector temperature is at most slightly lower than that of the geothermal probe.
  • Otherwise, the heat from the geothermal probe is extracted as with normal geothermal heat pump heating.

Because the heat is temporarily withdrawn from the collectors, the ground is cooled down less, and when it is withdrawn from the ground later, the heat pump achieves a higher coefficient of performance due to the higher temperature. On the other hand, excessive cooling of the collectors, which could lead to condensation and icing, can be avoided by switching to the geothermal probe in good time. (The switchover criterion may include factors that decide about condensation.) An additional low-temperature storage tank is not necessary because the geothermal probe fulfills this function.

Such a combined system is of course more expensive than a pure geothermal probe heat pump heating system, but enables lower primary energy consumption. In principle, the length of the geothermal probe (s) can be reduced a little to save costs, but if you go too far with this you endanger the energy efficiency again.

Systems with gas adsorption heat pumps

Systems that contain a natural gas-operated adsorption heat pump are a special case. Here, in turn, the heat pump can increase the yield from the solar collectors. This approach is an energy-efficient alternative to pure gas heating and is also more efficient than the conventional approach of a parallel combination of gas heating and solar thermal energy. In contrast to the systems with electric heat pumps discussed above, however, there is no option here to completely avoid the consumption of fossil fuels by using green electricity. After all, the gas consumption can be reduced depending on the circumstances (collector area, required flow temperature, climatic conditions, etc.) B. Reduce by a third compared to a pure gas heating - significantly more than with the conventional combination of gas heating and solar thermal.

Comparison of the systems

The comparison of the variants of solar heat pump systems described above is not trivial, as the achievable energy efficiency depends heavily on the respective conditions (e.g. climatic conditions, collector size and required flow temperature) and some variants can only be implemented under certain circumstances ( because they require a geothermal probe, for example). However, an approximate classification can be made as follows:

When it comes to capital expenditure and energy efficiency, different approaches differ considerably. The respective conditions also have a great influence.
  • A parallel system can allow a somewhat lower primary energy consumption than pure heat pump heating, but the solar share in winter remains small if you want to avoid large summer surpluses. A special case would of course be an application with high summer heat demand such as B. a swimming pool heating, where a heat pump only has to cover a few “critical” days.
  • Serial systems can significantly increase the proportion of heat produced by the solar collectors, as they considerably increase the collector's yield, especially in the cooler seasons. On the other hand, the majority of solar heat production then also requires the use of the heat pump. This raises the question of whether it would not make more sense to use another heat source for the heat pump (e.g. a geothermal probe). However, where a geothermal probe cannot be installed, the serial system with low-temperature storage (ice storage) is an interesting option with relatively high investment costs, but lower energy consumption than e.g. B. with an air / water heat pump and without the need for additional heating.
  • A combined system with collectors and geothermal probe is likely to be more efficient than a pure geothermal probe system, albeit with higher investment costs.
  • The gas adsorption heat pump in combination with solar thermal energy is a good alternative to pure gas heating or to the conventional combination of gas heating and solar thermal energy. However, it does not allow foregoing fossil fuels.

In principle, air / water heat pumps can also be used instead of geothermal probe systems, but they have a low energy efficiency, especially on cold days with high heat demand. The combination with solar collectors hardly solves this problem, as they contribute little on critical, cloudy and cold days.

Actual solar heating is usually implemented without a heat pump.

An alternative to all of the approaches mentioned is solar heating with a large seasonal heat store and additional heating only for a few cloudy and cold weeks of the year. Of course, this is only practicable for buildings with excellent thermal insulation and, if possible, large hot water tanks integrated in the building. If the additional heating were based on a heat pump, it would in principle be a variant of the parallel solar heat pump system mentioned above. However, an inexpensive boiler is more suitable for this reserve function.

Installation of the solar panels

The shading of collectors by trees or buildings is often a problem, especially in winter due to the low position of the sun!

Since a solar heat pump system should deliver a lot of heat all year round, it is particularly important here that the solar collectors are not shaded even in winter (when the sun is low), e.g. B. through tall trees.

It also makes sense to mount the collectors as steeply as possible in order to increase the proportion of winter production. (A certain loss of efficiency in summer is easy to get over.)

In systems with a large low-temperature storage tank, the collector area can be chosen to be quite large, as summer surpluses can then be easily stored. In this way, the solar proportion of heat production can be increased.

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See also: solar collector, solar thermal, heat pump, solar heating, ice storage
as well as other articles in the categories of renewable energy, basic terms, building services, heating and cooling