District heating nets for heat supply are very common in cities of the CIS. In contrast to the EU countries some district heating nets in the CIS are constructed as an open-circuit system regarding the hot water supply (Budig et al. 2008), e.g. in Bishkek, the Capital of the Kyrgyz Republic. In such systems hot water itself is delivered to the consumers via the district heating net, whereas in closed systems only heat is supplied. A simplified scheme of an open-circuit system of the combined heat and power plant (CHP) in Bishkek is shown in Fig. 1. This district heating net supplies 350,000 inhabitants in the city center with the base load of around 2,600 m~3 of hot water per hour and the peak load up to 3,000 m~3/h. Beside this CHP in Bishkek there are over 60 boiler houses with much smaller open district heating nets for remote city areas. The low water inlet temperature of 12°C, high base load and continental climate (= high solar irradiation and hot summer) in Bishkek are very favorable for preheating of the water using solar energy. For this purpose uncovered solar collectors can be applied very effectively reaching very high annual solar gains of over 1000 kWh_(th)/m~2a and low solar heat costs of less than 1 Euro-Cent/kWh_(th) (Vajen et al. 2008). Uncovered collectors can, however, cover only up to 20% of the heat demand as the outlet temperature is limited. Covered collectors, e.g. flat plate collectors, can be used to achieve higher temperatures and, thus, higher fuel savings. However, they are economically less effective than uncovered collectors because of the higher investment costs, so that a combination of both collector types is economically more feasible if higher temperatures should be achieved. The extraordinary low water inlet temperatures of district heating nets in Bishkek open more technical options. As the ambient temperature T_(amb) in summer (up to 40°C during the day and 20…25°C during the night) is predominantly much higher than the water inlet temperature T_(in), it is possible to preheat water with the ambient air using fin-and-tube heat exchangers (Frank et al. 2006a), Fig. 2. By this, heat gains can also be achieved during the night. Fin-and-tube heat exchangers (Fig. 3) are widely used for heat transfer between a liquid and a gas in industry and in the residential air conditioning. They have low investment costs and a very compact construction. For water preheating under the climate conditions of Bishkek the air-to-water heat exchangers have similar economical effectiveness as uncovered collectors (Frank et al. 2006b).
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