Technology: Energy Saving and Optimisation of District Heating
Energy Saving and Optimisation of District Heating
In many district heating systems as well as in district heat usage, there are large potentials for energy savings. Optimisation of district heating networks and use can both save energy and make district heating better fit to the future. The network optimisation typically includes reductions of the temperatures in the network, both forward and return temperatures.
Some network optimisation measures are:
- Minimise forward temperatures to enable lower network losses and higher efficiency of heat sources that are temperature sensitive, such as heat pumps including use of waste heat, geothermal heating, CHP plants. The forward temperature should be set at the minimum for safe production of hot water to avoid risk of legionella at the hot water for the consumers. This require a temperature of 55’C in hot water tanks. To enable that, forward temperatures at consumers shall be 58-60’C. With an efficient district heating network, the forward temperature from the heat supply station can then be 65’C. Often the temperature in colder parts of the winter should be increased above 65’C to ensure comfortable space heating. Is is important to regulate forward temperatures the minimum needed for space heating and for hot water production. If this is done with an automatic regulation with weather compensation, it is important that the regulation is properly adjusted to give the optimal temperature at all times, depending on outdoor temperatures.
- Minimise return temperatures with better regulation at heat user installations. This also reduces network losses and increase efficiency of district hearting supply with heat pumps and CHP, as well as with boilers that are equipped with flue gas condensation.
- Minimise flow of district heating water though hot water tanks with adjustments of valves, and replacement simple valves with thermostatic valves on the pipes from hot water tanks to district heating return pipes.
- Ensure a balanced distribution of district heating water to all radiators to combine comfortable indoor temperatures with a large difference between forward and return temperatures.
- Better regulation of shunt valves, both inside houses and in district heating networks.
- Better regulation of shunt valves at end points of district heating networks. Most district heating networks have valves that shunt water from forward to return pipes to ensure sufficient temperatures at all points in the network. If these valves are not already equipped with thermostatic controls, they should have thermostatic controls. The thermostats should be set at a minimum acceptable comfort temperature, typically 58-60’C, and they should be inspected regularly to avoid excessive flow through them.
- Insulation of pipes and substations.
- Replacement of pipes that are badly insulated.
- Disconnecting or changing network for consumers that have small consumption and are served by very long pipes with high energy losses. Changing networks can for instance be by replacing large pipes with smaller pipes or divided the heating network, so a small group of remote consumers are served by a decentralised heating station. This can be relevant in areas where some large consumers have stopped consumption, as closed industries.
Some energy saving measures in houses are:
- Insulation of pipes and hot water tanks.
- Optimal balance of heating to ensure that all radiators have equal flow. Use flow limiters that are often in-built in thermostatic radiator valves. Use also flow regulators in the network.
- Regular control of thermostatic valves of radiators and introduction of thermostatic valves where missing.
- Weatherisation of houses to reduce drought.
- Insulation of outer surfaces of buildings including windows, ceilings of top floors, ground floors, and outer walls, in particular behind radiators.
- Ensure a good air circulation around radiators.
With better insulation and balancing of heating systems, existing radiators can keep a house warm with lower forward temperatures from district heating. If district heating companies want to reduce temperatures, it can help consumers to save energy. Special attention can be given to consumers with problems, as consumers at the end of lines, consumers with especially problematic heating systems. This can avoid that a single house makes it impossible to reduce temperatures in a larger network.
Examples
Energy Savings by Sønderborg District Heating
The district heating company in Sønderborg, Denmark with 27.700 inhabitants and 15.000 district heating consumers (each consumers is a family, a company etc.) has invested in energy savings since 2006. The district heating company is progressive, but the magnitude of the energy savings were decided by Danish state regulation for energy savings. The annual new savings varied from year to year from 560 MWh to 3800 MWh. In each year the savings in the district heating system varied from 5% of total savings to 20% with the rest coming from savings by consumers. Savings in the district heating system was mainly realised with exchange of old pipes to new, better insulated pipes and insulation of installations in substations.
Consumer information by Hjørring District Heating
The district heating company is distributing consumer information to its consumers on how to save energy, minimise return temperatures and get most value of the district heating. The information is online here hjvarme.dk
Technical parameters
Savings: Losses in district heating systems depends on forward and return temperatures as well as ambient temperatures. As ambient temperature can be used the annual average temperature. This is 8’C in much of Northern and Eastern Europe including countries like Denmark and Ukraine. The heat loss is depending on the difference between the average of forward and return temperatures and the ambient temperature (the loss is of course also depending on the insulation of pipes and installations). If the forward temperature is 70’C and return 40’C, then the average is 55’C. With ambient temperature 8’C, the difference is 47’C. Each 1’C reduction in forward or return temperature (annual average) will then reduce heat loss 0,5/47= 1%.
If the heating is provided by a heat pump, each degree reduction in forward or return temperature (average weighted with heat demand) will increase efficiency of heat pump around 0,8% (if original COP of heat pump is 3,0, with 1’C reduction of forward or return water, the COP will then be 3,024). This is an average, different heat pumps have different temperature dependencies.
If the heating is provided by a CHP plant, reduction in forward and return temperatures will increase power production.
If the heating is provided by a condensing boiler, reduction in return temperature will increase efficiency.
Financial parameters
New pipes for replacement or new networks (material costs only):
- Up to 50 kW capacity: 42 €/m
- 50-250 kW capacity: 53 €/m
- 250 kW – 1 MW capacity: 69 €/m
- 1 – 5 MW capacity: 96 €/m
The total costs include material costs and installation costs. Installation costs depends on expenses of digging and restoring existing surfaces, layout of district heating network, as well as of wages and other costs in the country in question.
Sources for information
Costs of new and replacement district heating
Cases
Sønderborg District heating:
Hjørring District Heating
Technology suppliers and installers (coming)
- Thermostats and other control equipment: danfoss.com
- District heating pipes etc. logstor.com