HOT|COOL NO. 2/2019 - "Smart Heating System Integration"



By Lars Sønderby Nielsen, Managing Director, Enexio Solutions

Absorption technology offers a sustainable and well proven technology for district heating and cooling. With water as natural refrigerant, the technology offers GWP (global warming potential) value at 0 and a low CAPEX (capital expenditure) that gives a favourable payback period. Large heat pumps are a key in the green transition of district energy. The overall value of heat pumps is that they can utilize waste energy at low temperatures for usable energy at a higher temperature. Key challenges for all heat pump projects are to identify adequate low temperature heat sources and at the same time to ensure that the necessary energy source for operating the heat pumps is sufficiently sustainable. In addition, the legal frameworks and market demand are increasingly pushing for natural refrigerants. WHY THERMAL DRIVEN HEAT PUMPS? As an alternative to compressor driven heat pumps, absorption heat pumps and chillers offer several features that are attractive for district heating projects. The machine is ideal for utilising heat at low temperatures, thus recovering energy that would otherwise be wasted. Unlike a conventional heat pump, the absorption heat pump is fuelled by thermal energy, resulting in minimal electricity consumption and very low operating costs. In a heating plant, the necessary thermal heat source is often "freely" available since the heat used in the absorption pump is delivered back to the district heating system. The machine uses water as refrigerant, which means that the GWP value is practically zero. The technology is mature and well-proven, with hundreds of thousands of commercial installations worldwide. PROVEN TECHNOLOGY An absorption heat pump basically consists of 4 large heat exchangers, which together act as a thermal heat pump. The machine uses water as a refrigerant and a salt (LiBr) to absorb water vapour at low pressure. The fundamental working principle is that the refrigerant water evaporates in vacuum (0.8 kPa) and removes heat from the low temperature energy source. A LiBr solution mixture absorbs the refrigerant vapor and maintains the necessary low pressure in the evaporator chamber. This solution is then pumped to the generator where the refrigerant is revaporized using a high temperature heat source. The refrigerant-depleted solution is then returned to the absorber via a throttling device.

Figure 1. Operating principle of an absorption heat pump and chiller

MULTIPLE APPLICATIONS Heat pumps increase efficiency and improve the economy regardless of the type of production plant and available fuel. When establishing large solar heating systems, absorption heat pumps are used to cool the district heating return from the consumers before being sent back to solar panels and seasonal storage. The absorption heat pump can also be used to utilize surplus heat from nearby industry or data centres. In the Danish market, the current most common application is for flue gas condensation in biomass and waste incineration plants, where the driving energy for the heat source is freely available. Future development of geothermal energy sources is also expected to include absorption heat pumps for upgrading ground sources temperatures to applicable district energy levels. COP IS NOT ALWAYS THE MOST IMPORTANT FACTOR The heat pump COP is a measurement of efficiency and thus always to be considered. For practical applications, the heating COP (COP = heat output/heat source input) is equal to 1.7 for absorption heat pump applications. In direct comparison, this COP is low compared to mechanical chillers. Nonetheless, absorption can substantially reduce operating costs because they can be driven by low-grade surplus heat.

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