In a heating plant, the heat source energy can often be found as hot water from existing boiler installations. In such case, the heat source energy is in principle “free”, since the energy used for the absorption process is fully returned to the district heating system, only at lower temperature. In such cases, the heating COP is less important. Focus should rather be on exploiting the low temperature energy source to its maximum.
Figure 4: Jægerspris Kraftvarme in Denmark has installed 3 MW absorption cooling for increasing the efficiency of its solar heating plant.
DISTRICT COOLING Using absorption for district cooling purposes is the ideal way of upgrading low quality waste heat in the summer period. The operation principle of an absorption chiller is the same as for an absorption heat pump, only with the difference that it is the chilled water from the evaporator of the machine that is utilised and distributed to the consumers. When working as a chiller only, the energy from the absorber/condenser circuit is deposited in a cooling tower. District cooling is largely used already in for example Sweden, Finland, Germany and France. In Gothenburg, Sweden, for example, absorption chillers are widely used for district cooling purposes, converting surplus heat from waste incineration at typically 90°C to district cooling that can be delivered at 6°C to commercial building connected to the cooling network of the city.
Figure 2: The new biomass fired District Heating plant in Grenaa, Denmark, with a boiler capacity of 2 x 15 MW has installed 2 absorption heat pumps for flue gas condensation each providing additionally 2 MW heat.
FLUE GAS CONDENSATION For energy efficient district heating boilers, the first step is to install adequate economizers to cool down the flue gas and extract most possible energy. However, this will only cool down the flue gas to a few degrees above the district heating return temperature, meaning that a large amount of energy is wasted and discharged through the stack, often at about 50 °C or more. With an absorption heat pump, the flue gas can be cooled down to temperatures typically below 20 °C and in the best case all the way down to 10 °C, which means that the last residue of available energy can be used in the district heating.
Figure 5: Göteborg Energi in Gotenborg operates a large district cooling network, where 3 absorption chillers contribute with 12 MW cooling.
Figure 3: Basic energy balance in an absorption heat pump used for flue gas condensation
COMBINED HEATING AND COOLING SOLUTIONS If there is a simultaneous heating requirement, the chilled water circuit of the evaporator can be used for cooling at the same time as the absorber/condenser circuit is used for heating. An example of such a solution has recently been installed at the new University Hospital, DNU, in Aarhus, Denmark, where an absorption chiller delivers 3 MW of cooling to the hospital’s new centre for particle therapy. During summer operation, the waste heat is disposed of in cooling towers, while the waste heat in winter can be utilised for heating in the hospital’s local district heating network. The absorption chiller is powered by hot water at 105°C from the district heating transmission network. This converts 4.3 MW heat to 3 MW cooling and 7.3 MW district heating, which are used simultaneously.
SUMMER UTILISATION WITH SOLAR AND WASTE HEAT While many boiler installations with auxiliary heat pumps are out of operation in the warm summer period, absorption still finds its place with both solar heating and district cooling installations. For solar heating plants, absorption heat pumps can be used for optimisation of the solar field output. This is done by cooling of the district heating return temperature before it is reheated by the solar field heat exchangers. Such installations can yield up to 30% of increased performance from the solar panels. The feasibility of course needs to carefully examine the nature and cost of the thermal heat source required for the process.
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