A Swedish engineer has come up with an answer to making district heating more climate-friendly and found ways to help it save energy even at times of peak usage.
District heating is a common for heating buildings and hot water in many Swedish cities. In fact, every city with more than 10,000 inhabitants has a district heating network. It provides 60 per cent of the market share total and 90 per cent market share for residential apartments. The heat is largely produced from residual products from forestry, household waste incineration, or excess heat from industries.
But the demand for district heating varies greatly during the day. For example, it substantially increases in the morning when many people take a shower at around the same time. To cover the increasing demand, many district heating producers are forced to temporarily run fossil fuelled reserve boilers, known as peak load boilers.
In Gothenburg, the peak load boilers are fuelled by natural gas.
Now Johan Kensby, a PhD student in building services engineering, has shown how it can be avoided in a cheap way. His solution simply stores heat in the buildings that are already connected to the district heating network.
“Buildings have large thermal mass. Heat can be stored in the floor, walls, ceiling, and in the water in the radiator system,” he said.
In a pilot study, he investigated how the indoor climate is affected if the district heating system boosts radiator heat at certain times and reduces it at others. He found that it is possible to store as much as 0.1 kilowatt hours of heat per square metre of a building without the indoor temperature varying by more than 0.5 degrees Celcius.
Such a small variation in indoor temperature goes unnoticed by residents. Temperature variations of this size happen all the time, for example when cooking, yet done in this controlled way can provide significant savings.
It is possible to therefore even out the load on the heating system. Faced with an expected peak, such as the morning rush to the shower, the system can prepare by heating the radiators in the buildings a little extra for a few hours in advance. A five-minute shower “costs” about two kilowatt hours. An apartment of 80 square metres can therefore store the heat energy equivalent of four showers.
Water heating is the largest source of greenhouse gas emissions from an average Australian home. It is the second largest segment of household energy use in Australia, after space heating and cooling. It accounts for about 21 per cent of the energy and generates about 23 per cent of the greenhouse gas emissions (DCCEE 2010).
In Australia, about 48 per cent of the energy used for water heating comes from natural gas, 45 per cent from electricity, three per cent from liquefied petroleum gas (LPG) and four per cent from solar (DCCEE 2012). Electric water heaters in particular contribute to these emissions. Only half of Australian homes use electric water heaters, but they contribute 80 per cent of hot water greenhouse emissions.
A 10-minute shower using a 1 star rated shower head will use approximately 200 litres of hot water.
A 10-minute shower using the most efficient 3 star rated shower head will use approximately 75 litres of hot water.
In order to heat just one litre of water by one degree takes 4,184 joules of energy
In Kensby’s solution, when the shower rush comes, the heat pumped into the radiators is redirected to heat the shower water instead. Meanwhile, the buildings keep a nice indoor temperature by using the stored additional heat. When the shower rush is over, heat is directed into the radiators again. Thus, the heat load peak is cut and the district heating supplier can avoid running the peak load boilers.
Kensby has further simulated the effects on a much wider scale. He has considered the outcome if his method of heat storage in buildings was introduced into Gothenburg’s district heating network.
The results show that the daily variation can be halved if the 500 largest substations of the total of 20,000 substations, are used for heat storage.
“It would reduce the district heating carbon footprint drastically. It would also reduce production costs and thus increase the profitability of district heating,” he said.
Tim Edwards, from the Australian Refrigeration Association, however, says Australia has a way to go to benefit from this approach. It has been slow to embrace the opportunity of district energy, despite clear examples of success here and substantial proof available across Europe.