Cogeneration: size matters Big cogeneration systems have higher electrical efficiency compared to small systems. They are significantly cheaper per kW installed. But they need a thermal sink big enough to take up several 100 kW up to several MW. This can either be found in industry or a district heating system that brings together many consumers. For years cogeneration systems were designed to cover as much of the base load as possible. With raising fractions of PV and wind in the electricity system, the need for flexible power supply increases: the power output of engines and gas turbines can be adjusted very quickly to the changing needs. Combined with sufficient heat storage capacity, cogeneration forms the backbone of the electricity system. The high capital costs of small systems do not allow this flexible operation.
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Very often a common supply is more efficient. Coordinated approaches of communities have a stronger effect on the climate strategy compared to various decisions of individuals. Local energy supply concepts that include a local heat strategy are a good chance to check the possibility to supply heat via district heating systems. District heating systems are a part of local infrastructure with a long lifetime and thus need a long engagement, a stable structure of the heat supply company and a stable political support. Prejudices and barriers District heating has to fight against a whole bunch of prejudices. Of course, not every possible building block or village can be served by district heating, but for many types of settlements good solutions can be found. The obstacles and open questions should be discussed very thoroughly to end up with a robust solution based on broad support. Very often we hear that district heating only works in city centers with very high density of heat demand. Many examples, meanwhile, prove that even significantly less dense structures can be supplied economically. Energy losses of the systems are a crucial factor: If during the whole lifetime of the system, 20 % or more of the energy delivered to the system is lost under way, the advantage of high efficiency achieved in the boiler house dwindles. While designing a district heating system, the heat losses have to be calculated considering various scenarios of customers connecting to the grid. The effect of improved thermal insulation in existing buildings has to be considered as well. Our calculations show that this effect is strongly overestimated: The real refurbishment rate is still around 1 percent per year. Even a doubled retrofitting rate hardly ever is a reason not to choose to install a district heating system. On the other hand, district heating must not lead to a slowdown in the ambitions for retrofitting of the dwellings. So-called bio energy villages with a very low energy density are often only an economical solution because usually the heat supply is calculated with almost no cost. This will prove difficult in the long run. One argument frequently heard is that home owners prefer decentral heating systems due to the higher safety of supply. Some fear the stability of the local heat supply company and
Advantages for the customer The customer connected to a district heating system enjoys greater ease of mind as he does not have to look after his heating system. This includes operation, maintenance, chimney sweep, purchase or storage of fuel and the regular replacement of the heating system. If the district heating company offers to maintain the heat transfer unit, the customer has even less to care for. The space required for a heat transfer unit is minimal. In multi-family houses, the district heating company can even bill the tenants directly. The safety and efficiency of the energy supply increase, since in a central boiler house additional energy sources can be installed much easier compared to decentral approaches. Advantages for the city The safety of the energy supply is an issue for the municipality as well. Especially systems using significant fractions of locally supplied renewable energies keep the money in local circuits. Fossil fuels are purchased abroad - often in politically unstable regions. Cooperative structures lead to additional local value. Heat supply is an additional element of a local infrastructure. Like the water supply, the sewage system or electricity supply: Nobody sees advantages in a decentral approach.
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