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How do hybrid heating systems work?

In modern homes, not only traditional but also alternative thermal energy generators are increasingly being used: such hybrids allow significant savings on heating. But before deciding to purchase such equipment, you need to understand how it works and in what case its use would be appropriate.

Solar water heaters on rooftop.

Boiler + solar collector 

The basis of the system is a set of vacuum-type solar panels and an electric or solid-fuel boiler. It starts at the moment when solar energy is no longer enough for full heating and hot water supply (DHW). Important components are also a storage tank with a built-in heat exchanger, a circulation pump, and an automatic control unit.

Principle of operation

All system components operate in such a way that they complement each other when necessary. Exactly as much heat is produced as is needed. 

By absorbing heat from the sun, the panels heat the antifreeze circulating in the collector tubes. Entering the heat exchanger, in turn, heats the water. The temperature of antifreeze and water is set manually and then adjusted automatically. The heated water accumulates in a tank, the volume of which, depending on the size of the home, can reach up to 3000 liters (Fig. 1).

Scheme of operation of a heating system based on a boiler and solar collector. 

Next, the storage tank distributes water along two circuits at once – heating (warm floors) and hot water supply. This can be done with the special design of a heat accumulator, assembled according to the “tank in a tank” principle: thanks to it, the water inside the tank is divided into that which will be used for heating and that which will be needed for household needs (showering, washing dishes, etc.)

If there is little sun and its heat is not enough (in winter, as well as at night and on cloudy days) – the computer unit automatically turns on the boiler. After heating in the boiler, the further passage of water occurs along the same chain. Thus, the system is completely autonomous. The need for human intervention in its operation is almost eliminated. The savings occur because 70% of the required thermal energy will be generated by the solar collector.

Most often, solar collectors are mounted on the roof, but in some cases they can also be placed on the ground

Advantages and disadvantages 

The system runs on solar power almost all day, so the need for electricity is minimal. Especially in the summer, when there is no need to turn on the heating. Electricity is used only to operate the circulation pumps and the control unit. The boiler is connected mainly at night, and at this time the cost of electricity supply is the lowest. If the boiler is not electric, then you can save on solid fuel, which is also not very cheap.

However, there are also disadvantages. Firstly, the system works ideally only in regions with a large number of sunny days a year. Where the weather is often cloudy, the savings will not be so noticeable.

Secondly, in case of frequent power outages, you will have to use a generator or install solar panels, which entails additional costs. Thirdly, fully automated control requires professional maintenance, and this also costs something.

Boiler + heat pump 

In this scheme, the heat pump completely replaces solar panels. Its design includes a geothermal circuit placed in the ground, as well as a compressor, condenser, and evaporator, located in a separate unit inside the house (Fig. 2).

The evaporator and condenser act as heat exchangers. The same electric or solid fuel boiler is used as a backup heat source. There is also a storage tank. Control is carried out in automatic or semi-automatic mode.

Fig.2. Heat pump with horizontal geothermal circuit. Such an arrangement of the heat transfer system will require expensive excavation work. 

Principle of operation

Located at a depth below soil freezing (2.5-100 m), where the air temperature does not fall below 5-70 C°, the pump circuit absorbs underground heat. The resulting thermal energy is transferred to the antifreeze circulating through the circuit, which transfers it to the evaporator. Next, the refrigerant (freon) comes into play, taking heat from the evaporator and entering it, then into the compressor. As a result of compression in the compressor, the refrigerant heats up and enters the condenser, and there the heat is transferred to water. Coming out of the condenser heated to 60 C°, the water first ends up in the storage tank, and then in the heating appliances. Temperature sensors monitor the heating of water: if it does not heat up enough (for example, in severe frost), they immediately send a signal to the controller, after which the electric boiler automatically turns on.

Advantages and disadvantages

A heat pump can be used not only for heating but also for cooling: in the summer, using a reversing valve, the device will remove excess heat from the room.

The efficiency of a pump is determined by the ratio of the amount of energy required for its operation to the amount of heat generated. For geothermal installations, this coefficient is equal to three. That is, by spending 1 kW of electricity, the pump supplies 3 kW of thermal energy.

The device is safe since its operation does not use either open fire or flammable substances. The main disadvantage is the high cost of the equipment and its installation.

However, installation costs will be two times less if we are talking about a device that will take heat not from the ground, but from a reservoir: in this case, excavation work may not be carried out. But for this, you need to have a pond or lake right next to the site.

Heat pump + solar system

The system includes solar panels and various elements necessary to operate the heat pump. Fan coil units and heated floors are used to heat the air. The accumulation and distribution of water for heating and hot water occurs in a heat accumulator. Heat transfer in the circuits of a geothermal installation and the circuit of a solar system is carried out differently: in a solar system through a separate heat exchanger (buffer), and in a heat pump using the “evaporator-compressor-condenser” chain. Both the collector and the pump are combined into a single circuit in such a way that they can function both together and separately 

(Fig. 3) .

Scheme of operation of a heating system based on a heat pump and solar collector. 

Principle of operation

In summer, the heat pump works mainly for cooling, and solar collectors heat water for domestic purposes. In late autumn and winter, depending on the weather, the devices are most often turned on alternately: during the day – collectors, at night – the pump. If there is little or no sun, heating occurs only with the help of geothermal equipment. As in other hybrid systems, the temperature of water heating and the sequence of turning on the units is controlled automatically, and the movement of liquids along the circuits occurs due to circulation pumps.

Advantages and Disadvantages 

The presence of two complementary devices – solar panels and a geothermal installation – allows you to use electricity in very small quantities. Thanks to natural renewable heat, a comfortable temperature is maintained in the house both in winter and summer, which does not require any serious annual costs. However, the initial cost of this system is also still too high. For the same reason, the payback equipment can take 10 or even 20 years. In addition, to operate such a hybrid in regions with a cold climate, you will have to stock up on a backup heat source (electric boiler), which will increase not only the initial investment but also the annual heating costs.

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