Cold District Heating and Heat Pumps

The term 'cold district heating' sounds like an inner contra­diction, but it describes a system for transporting energy in the form of heat at a low temperature level. It is precisely this, in conjunction with heat pumps, that can provide a CO₂-minimized heat supply in urban areas.

In the past decades, district heating was often used in cogeneration to distribute the unavoid­able waste heat from thermal power generation in fossil-fired power plants as heating for buildings. In this way, the low degree of utilization of primary energy of about one third in old and about half in new thermal power plants could be significantly increased, at least during the heating period.

However, as part of the necessary measures to reduce CO2 emissions, these thermal power plants will have to be taken off the grid and can at most be used as reserve power plants with low operating times. Cogene­ra­tion and the waste heat distribution function of district heating thus become obsolete.

On the other hand, fossil-fuel boilers will also have to be replaced in the future by heat generators with low CO₂ emissions. Heat pumps powered by regeneratively generated electricity are a particularly suitable concept for this. However, the function of a heat pump also requires an external heat source. In conventional systems, this is either the ground or groundwater or the ambient air, from which heat is extracted via a suitable heat exchanger.

In rural areas with a low building density, it is rather unpro­blem­atic to install an appropriate heat exchanger. In densely built-up urban areas, however, undeveloped areas for heat exchangers in the ground are rare on the one hand, and on the other hand there is a lack of suitable installation options for air heat ex­chang­ers, especially since the latter can generate considerable noise, particularly if they are compactly built to save space.

This is where 'cold district heating' comes into play, with water flowing through the pipes at approximately ambient temperature. Heat pumps connected to the 'cold district heating' can use it as a heat source. To do this, heat must be supplied to the district heating network elsewhere.

The highlight of 'cold district heating' is that it can be used not only as a heat source for heating heat pumps, but also as a heat sink that absorbs heat given off by cooling heat pumps. If, at the same time, heat is taken from the grid for heating and heat is supplied from cooling plants, this leads to at least partial compensation

Due to the weather, the heating demand occurs mainly in winter and the cooling demand mainly in summer, i.e. not at the same time. With a large reservoir as seasonal storage, this can be bridged and a balance established.

Up to now, in Central Europe the annual average heating demand has outweighed the cooling demand. In the future, it is to be expected that, on the one hand, heating energy requirements will decrease as a result of improved thermal insulation standards, and that, on the other hand, in view of global warming and more frequent hot days, cool­ing will increasingly have to be provided in buildings. Presumably, annual heating cooling demands will therefore converge, which is advantageous for the operation of a cold district heating network with seasonal storage.

The potential of this approach is to be illustrated with a rough calculation: After closure of the Hambach open pit mine in the Rhenish lignite mining region, the remaining hole is to be filled to form a lake with a volume of about 3.5 km³. If this lake is heated or cooled by 2 degrees, this corresponds to a heat quantity of 8 TWh or approximately the annual heating energy demand of 1 million people, i.e. about the population of Cologne. This does not necessarily mean that the entire city of Cologne should be connected to the lake, but it does show that the heat capacity of the lake is sufficient as a seasonal reservoir for large parts of the surrounding cities of Cologne, Düsseldorf, Aachen, etc., if they are connected via a cold district heating network.

A future Hambach lake with a surface area of about 40 km² receives an annual solar irradiation of about 40 TWh, a large part of which penetrates the lake, is absorbed there and compensates for extracted heating. This can also happen if a floating PVT system is installed on part of the lake with collectors that both generate electricity and emit heat.

Not everywhere are such large storage volumes available as in the Rhineland. However, the combination of cold heat network with seasonal storage and heat pumps can also be scaled to smaller dimensions and can therefore be used in many places.

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by   D. Hennings • www.eclim.de •