Power plants using low-temperature energy sources. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Alternative energy sources

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Ground heat exchangers in vertical wells have been widely used in the last 10-15 years as a low-temperature heat source for heating and hot water supply systems using heat pumps. This environmentally friendly heat source is used quite often, for example, in Switzerland, where about four thousand such installations are currently in operation.

The Altai Regional Center for Non-traditional Energy and Energy Saving conducted research on the mutual influence of a vertical ground heat exchanger and a heat pump. The automated heat pump unit ATNU-10 (working fluid - R22) was taken as the basis, developed by AK "INSOLAR" within the framework of the State Scientific and Technical Program of Russia "Environmentally Clean Energy" and manufactured by the "ECOMASH" enterprise (Saratov). The system also includes a vertical ground heat exchanger in a well with a depth of no more than 100 m (as hydrogeological studies have shown, 67% of the population of the Altai Territory lives in the territory where the depth of the first aquifer is less than 30 m). The base soil temperature is assumed to be 280 K, which corresponds to the average temperature estimate at a depth of more than 5 m for the conditions of the Altai Territory.

The automated control system of the ATNU type heat pump is designed in such a way that it works under optimal conditions with a constant value of the heat flux determined by the heat flux from the primary heat source, the inlet temperature of the high-temperature circuit and the mass velocity of the heat carrier of the high-temperature circuit. If the required heat load decreases, the heat pump must be switched off until the set temperature is restored. If the power of the ground heat exchanger is insufficient to cover the heat losses in the high-temperature circuit, a peak closer must be switched on.

The results showed that the thermal energy extracted from the soil linearly depends on the logarithm of the working length of the heat exchanger. Under these conditions (filtration speed 10 m/day), to obtain 5-6 kW of thermal power from the ground, the required depth of the heat exchanger will be 50-60 m. The design features of the ATNU require certain conditions for the flow of coolant in the high-temperature circuit. The minimum coolant flow in the heating circuit should be 0,3 kg/s (1 m3/h). With smaller volumes, heat accumulation will begin in the system and, as tests on a full-scale installation have shown, this will lead to an increase in the temperature and pressure of the refrigerant, a deterioration in the operation of the evaporator and a decrease in heat removal in the ground heat exchanger. And although the temperature of the high-temperature circuit coolant increases, the efficiency of the entire circuit, determined by the heating coefficient, decreases.

Great interest in the use of soil as a source of heat is shown in Europe. The design of the evaporator is proposed in the form of a serpentine of tubes with a diameter of about 25 mm, laid at a constant depth over an area of ​​several hundred square meters. In order to reduce capital costs, the tubes are located as close to the surface as possible.

The study of soil as a heat source conducted in Europe showed that the heat flow to the evaporator from the soil is 20-25 W/m, the minimum value for Europe is 10 W/m, the maximum is 50-60 W/m.

The optimal depth and spacing of the tubes are 1,5 and 2 m, respectively. In some cases, due to mutual influence, the limit of 2 m is extended. The pipes can be placed at a shallower depth, but the heat pump performance can be reduced by 5% for every degree of evaporator temperature drop.

In addition to the option of evaporating the refrigerant directly, it is possible to use an intermediate heat carrier - brine circulating through pipes in the ground and giving off heat to the refrigerant in a special heat exchanger. The average brine temperature in winter is -3°C.

If the water content of the soil is high, the performance increases due to the increase in thermal conductivity and good contact with the tubes. A large concentration of gravel in the soil causes deterioration in performance.

In Denmark, the possibility of using not horizontal, but vertical tubes, which can be used in the mode of not only heating, but also cooling the building in summer, when a reversible heat pump is used, has been considered.

An interesting detail was also discovered. The minimum ground temperature is always higher than the air temperature and is reached two months later when the required heating output decreases.

Vertical tubes take up less space and allow some use of the heat stored during the summer months, which gives them an economic advantage. Studies of vertical U-tubes have shown the possibility of significant heat recovery. A horizontal evaporator from an area of ​​150-200 m allows you to get 12 kW of heat. U-tubes placed in wells with a diameter of 127 mm and a depth of 8 m made it possible to obtain 12 kW from only two wells. It can be seen from this that U-shaped tubes reduce the required soil surface by 10-20 times compared to horizontal ones.

Despite the comparative cheapness of domestic heat pumps compared to foreign ones given the current weak financial situation of enterprises, the introduction of heat pumps encounters certain difficulties. Not the least role is played by the great novelty and unfamiliarity of this technology for our consumers. These problems were overcome abroad by providing benefits for several years to enterprises introducing heat pump units. In most countries of Western Europe, a lower tax was established on profits received from the use of heat pumps, and in some countries direct financial subsidies were made. Thus, in Austria, financial subsidies of up to 100 thousand schillings were established for companies using heat pumps, and in Germany in the early 90s, such companies were given the right to a tax discount reaching up to 7,5% of capital costs (subject to their capitalization), which is equivalent to a financial subsidy of up to 20% of the costs of heat pump installations. As a result, 105 thousand fuel pumps are currently operating in Austria, providing annual savings of 116 thousand tons of fuel oil.

In addition to the use of ground heat, the most attractive for use in home heat pump applications is the "free" source of heat for creating comfortable conditions inside the house - air. It is publicly available and has attracted the most attention in mass production. Where water is available, it has several advantages over air. The use of waste heat or solar collectors is being actively explored, in which there is interest in both Europe and America.

The most widely used heat pumps with air as a heat source from the very beginning of their use in the home. Basically, air is also a heat sink. As a heat source, air has a number of disadvantages, so careful optimization of the design is required depending on the installation location, where the air temperature can vary significantly.

The performance of the heat pump, and especially the COP, decreases as the temperature difference between the evaporator and condenser increases. This has a particularly unfavorable effect on air source heat pumps. As the ambient temperature drops, the amount of heat required for heating increases, but the ability of the heat pump to maintain even a constant heat output is significantly reduced. To overcome this shortcoming, additional heating is often applied.

For the conditions of England and most European countries, the cost of a heat pump with any heat source is noticeably higher than that of a conventional central boiler. The larger the share of the heat pump in the domestic heat load, the higher the difference in investment, so heat pumps are usually calculated for only a part of the annual heat load, and the rest is provided by an additional heater, most often electric (in the USA) and fossil fuel (in Europe). The choice between them is determined by the ratio of capital and operating costs. If a heat pump also provides air conditioning in the summer, its size and power may be dictated by this particular application.

Additional heating is required when the ambient temperature drops below zero, and the heat loss of the building exceeds the heat output of the pump. In order to increase the economic efficiency of the system, the inclusion of an additional heater, in this case electric, is recommended only when the heat pump cannot cover the full load.

All heat sources for heat pumps are to one degree or another exposed to the influence of solar energy, but it can also be used directly using solar collectors with coolant circulation, heating the air entering the evaporator using solar concentrators. Although solar concentrators seem to be more suitable for absorption heat pumps. They are still little used at home, but are the subject of considerable research work. To preheat the generator in the absorption cycle, higher temperatures are required than those achievable by conventional flat plate collectors. However, the use of an absorption cycle for air conditioning allows heating from flat-plate collectors, since the temperature here must be lower and therefore air cooling is carried out in the summer, just when solar radiation is intense and the collector temperature is increased.

Together with other sources of heat for heat pumps, flat-plate collectors placed on roofs are widely used. In general, solar collectors are intensively studied for use not only with heat pumps, but also independently, as well as in circuits with heat accumulators. The latter are also of interest to heat pumps as a source of heat on cloudy days or at night.

By supplying heat to the evaporator at a temperature higher than the ambient air, ground or water, solar collectors increase the COP of the heat pump.

Usually the intermediate coolant - water transfers heat from the collector to the evaporator. But there may be a complete combination of the collector with the evaporator, where the refrigerant evaporates directly inside the tubes of the solar collector.

Often the heat from the solar collector is fed into a liquid heat storage where the evaporator tubes are immersed. The heat storage plays an essential role in any solar heat pump system. In a Phillips house, for example, a solar collector (20m2) collects 36-44 GJ of heat per year (with an average efficiency of 50%) stored in a 40m3 tank at temperatures up to 95°C.

A minimal energy house scheme was proposed using three heat pumps: one to transfer heat with increasing temperature from the solar collector to the battery, the second from the battery to the heating system, and the third from the battery to the hot water system.

Solar collectors are also considered in combination with ground ones. It has been established that the dimensions of the solar collector must be more than 3 m2 per 1 kW of heat loss from the home. With a solar collector with an area of ​​30 m3 with a ground evaporator occupying only 100 m, COP = 3,4 is achieved. If you use only a ground evaporator, then a surface of 300 m is required, and this results in COP = 2,7.

However, it may be that despite the increase in COP, the fuel savings may not justify the cost of the installation, especially the solar collector. Other works in this area show that with a thermal power of HPI of 6 kW, a surface of 20m2 is required.

In addition, HPP can use heat discharges from the housing itself, for example, exhaust gases from kitchen stoves or from the kitchen in general, waste water. In Holland, TN was applied to a domestic dish dryer. The heat of the ejected moist air is used to heat the dry air supplied to the dryer. The warm moist air from the dryer passes into the HP evaporator and is cooled down. When cooled, moisture falls out of it, and the air becomes suitable for recirculation. The evaporator uses both the sensible and latent heat of the outgoing air. The recirculating air passes through the condenser and is heated by the heat of condensation. Energy savings reach about 48%. The following are some characteristics of HPPs that are widely used abroad.

Table 2.1.2. Characteristics of the Carrier HP unit (USA) - a simple reversible air-to-air heat pump.

The characteristics of Lennox HP are combined with a fire heating system, which eliminates the additional heating system. Table 2.1.3.

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