Industrial heat pumps

heat pumps and district heating

The role of district heating and cooling systems for the supply of thermal energy (hot and cold) in urban areas will become increasingly important in the coming decades, because without the help of these utilities, the decarbonisation of energy consumption in areas with a high density of buildings will be very difficult if possible at all.

In fact, these networks for the transport of thermal energy make it possible to balance the demand and supply of heat (or cold) in a given geographical area, and, by doing so, satisfy a significant part of the demand for energy. For example, district heating and cooling networks make it possible to distribute waste heat, e.g. coming from industrial activities or produced in cogeneration, to those who need it in other areas of the city. These grids can also bring renewable energy (solar thermal, geothermal, biomass, etc.) produced in large quantities outside urban areas, to customers in the city centres.

District heating systems can be very efficient from an economic and environmental point of view and make it possible to significantly reduce the emissions of greenhouse gases or other air polluters.  

There are many ways in which heat pumps can be integrated into district heating and cooling systems and transform them into true “smart energy systems” for thermal energy. Here below some examples are given.

heat pumps as producers of heat

A heat pump makes it possible to feed waste heat which has a temperature below the one required by the network, into a district heating system, as shown schematically in the figure on the right side. If possible, the energy from the heat pump should be fed into the return line, in order to keep the DT over the heat pump as low as possible.

The heat pump can upgrade thermal energy from industrial plants, datacentres, underground stations, supermarkets, etc. in order to power the network. Also renewable energy (solar thermal, geothermal, etc.) can be used to this scope. Both solar thermal energy and geothermal energy are best produced at low temperatures and can therefore benefit significantly from the help of a heat pump. By powering the heat pump, at least partially, with electricity from photovoltaic plants a very significant reduction of CO2 emissions can be obtained.   

Heat pumps can be applied for different network temperatures, by choosing the appropriate refrigerant. As shown, the ThermBooster™ can easily supply heat up to 120 °C or higher so, also older networks can integrate these machines in a profitable manner.

Today, in Europe, there are two main trends:

  • where possible, attempts are made to reduce the temperature of the district heating network, above all to reduce the thermal losses from the piping but also to facilitate the integration of waste energy or renewable energy in the system.
  • district heating networks must increase the share of renewable- or waste energy in their final product in order to contribute to the decarbonisation of the energy used for heating and cooling of buildings,

In Italy, most networks have a forward flow temperature above 100 ° C. The return is usually around 70 °C. There are also networks that operate with lower temperatures, for example about 70 ° C for the heat  delivery and about 50 ° C for the return.

high temperature heat for users of a low temperature network

Lowering the temperature of the distribution network could create problems for certain users who need heat in the range of 80 – 110 °C or low pressure steam. The ThermBooster™ of SPH Sustainable Process Heat GmbH, allows to serve these clients as well, as is shown in the figure on the left side. Users in this category can be, for example, historic buildings whose heating systems require temperatures around 80 – 90 °C, hospitals (sterilisation), factories that need low pressure steam (laundries, food & beverage, pharmaceutical, etc.). Little heat is added to the system (only the electricity used by the compressor) but in this manner, the heat pump can improve the  efficiency of the network and contributes to the reduction of the  local emissions.

increased network capacity

A heat pump can increase the transport capacity of the network, by lowering the return temperature. This could, in certain cases, allow the connection of a new urban area to an existing backbone which is already used at full capacity. The concept is schematically shown in the flow diagram on the right. This solution adds no external energy to the system other than the electrical energy of the compressor but improves the efficiency of the existing network, reduces the specific energy for pumping and contributes to the reduction of the  local emissions.

increased thermal capacity of heat storage

District heating networks that use heat produced in cogeneration often include thermal storage systems. These buffers are used to bridge the difference in time between the maximum demand for thermal energy and the peak-load of the electrical network (peak-shifting). When the network temperatures are > 100 °C the specific costs for these systems can be rather high due to pressure constraints. A heat pump can easily double the useful capacity of such storage, as the image on the left side illustrates and can bring in this manner important economic benefits.

future developments

The proposal of 14/07/2021 of the European Commission for the new directive on energy efficiency (document COM (2021) 558 final), sets as an objective that from January 1st, 2050 all new and existing district heating systems that supply more than 5 MW of total energy, use “exclusively renewable energy and waste heat, with a share of renewable energy equal to at least 60%” (cf. art. 24).

From what has presented above it is clear that heat pumps can make a very important contribution to reach this ambitious goal.