Clean district heating and cooling system – how can it work?
The following concept for clean district heating and cooling consists of multiple renewable energy sources, co-generation of heating and cooling, as well as energy storage technologies in both the district heating network and in buildings. Already today, IT systems with smart control and automation functionalities can connect renewable energy units, storages, and buildings in a way that allows total phasing-out of fossil fuels from the heating systems of cities even in the Nordic region.
Figure: Fossil fuel-free district heating and cooling system
In the following section, the main components of the 100 % fossil fuel-free district heating and cooling system are described. The system is based on a high level of electrification. It has flexibility solutions to solve the seasonal, weekly and daily mismatch of energy production and demand, sufficiently high-temperature supply for the buildings, and balancing the electricity and heat markets.
Technologies and solutions
Heat pumps are power-to-heat conversion technologies. Their size can vary from industrial to various building-scale solutions. Heat pumps can simultaneously produce both heating and cooling and thus feed both heating and cooling networks. This is essential in today’s cities where many buildings, such as offices, schools, restaurants, cinemas and shopping malls, often require cooling also during the winter due to high internal heat gains. During summers, simultaneous heating and cooling are needed for sanitary hot water and space cooling.
Heat pumps consume electricity for approximately 15-50% of the heat or cooling produced. Also other parts of the system such as electric resistances, pumps and valves require electricity. Heat pumps produce fossil fuel-free heating, if this electricity is produced without fossil fuels, as is the case with wind, solar, hydro, nuclear or biomass-based power.
Industrial heat pumps
District heating can be supplied by industrial heat pumps, which can collect their main energy from the ground, lake, sea, air and from different excess heat sources like wastewater treatment plants, data centers, industrial processes, large bakeries, supermarkets and buildings. This allows the reuse of low temperature excess heat, which until now has mostly been wasted. For example, in the Finnish municipality of Mäntsälä, the excess heat from the Yandex data center produces after the ongoing upgrade up to 80% of the district heating annually.
Heat pumps in buildings
Various types and sizes of heat pumps can be installed in buildings depending on the end-use. The following examples illustrate the versatility of heat pumps.
Ground source heat pumps (GSHP) can be installed in shopping malls and large office buildings for supplying cooling and heating directly to the building during the opening hours. During weekends and holidays the heating and cooling can be produced to district networks.
Heat pumps for public buildings and housing companies can be designed sufficiently large to produce heat also to district heating and cooling networks in a two-directional way. In this case, the boreholes can also be used as heat storages, storing heat during the summertime cooling.
Exhaust heat pumps can be installed in old buildings, where the ventilation system has only outtake channels, as an auxiliary heating source. Air to air heat pumps can be installed in residential buildings, nurseries, stores etc. for cooling and auxiliary heating. Air to water heat pumps can be installed in all kinds of buildings for water heating.
Wind and solar power
The electricity needed by the heat pumps can be produced mainly by wind power. During the summer when cooling is needed, solar photovoltaics (PV) can be used to feed electricity to the heat pumps. Solar PV panels can be installed both in public and residential buildings mainly designed in a way to allow the use of the produced electricity on site. Also, utility-scale installations can emerge following the trend of declining solar PV prices and evolving business models around solar energy utilization.
Heat storages and demand response automation
Heat storages are essential in the fossil fuel-free district heating system due to the variable wind and solar power production, and the fluctuation of electricity prices. These variations have different rhythms – from seconds to months – and therefore different storages and demand response solutions are needed.
Large and centralized heat storages, as well as short-term heat storages in buildings, can be charged when the electricity price is negative or very low for example during peak wind power or solar energy production periods. The heat storages are then discharged during peak heat and electricity demand times. Earth pits, old oil caverns, borehole fields, large water tanks etc. can be used as heat storages offering a significant backup potential. Short term heat storages in buildings and electrical storages with demand response automation will allow renewable energy to be shifted several hours, solving the daily mismatch of demand and production curves. Large heat storages can solve the mismatch of demand and production curves during days, weeks or even months.
Solar heat collectors
Additional heat sources for the clean district heating system are solar heat collectors, which are most favorably built as plants next to the seasonal heat storages in places where the value of the land is low. Solar heating plants can charge the heat storages during summer time. The solar heat can also be directly delivered to heat pumps supporting them to reach sufficiently high temperature for direct supply into the network.
Combined heat and power (CHP) production fuelled with biomass
During the low wind power production moments, dispatchable backup power and heat production is needed. Bio-based heat and power production (CHP) operates in those moments when also the price of electricity is high since the price is based on the marginal cost of the currently running most expensive production method. When the electricity price is high and heat demand is low, then the excess heat also from the CHP plant can be stored in the heat storages. This way heat storages can maintain and improve the overall efficiency of the CHP plants when it takes turns with variable wind production. The system efficiency has in principle its maximum when all heat is produced with either heat pumps, CHP or by utilizing excess power or heat.
District heating and cooling network operators need automation systems to connect and control the production and storage units as one virtual power plant. For this, various IT solutions are already commercially available and new solutions continue to develop.
Principal scientist Francesco Reda, VTT Technical Research Centre of Finland, email@example.com, +358 40 8403680
Stakeholder Relations Director, Researcher Karoliina Auvinen, Aalto University, firstname.lastname@example.org, +358 50 4624727
Samuli Rinne, Karoliina Auvinen, Francesco Reda, Salvatore Ruggiero and Armi Temmes. 2018. Discussion paper: Clean district heating – how can it work? (pdf). Publication of the Smart Energy Transition project funded by the Academy of Finland’s Strategic Research Council.
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