In this module, we’ll have a closer look at 4th generation district energy and go through the main principles behind these sustainable systems. District energy has provided reliable and cost-efficient energy since its humble beginnings in the late 1800’s. Today, centralised production of heated or chilled water is one of the keys to overcoming the challenges of rapid urbanisation and climate change. District energy, however, is still evolving and intense research has paved the way for so-called 4th generation district energy – an innovative energy eco-system with great potential. But what separates this new generation from the current one?

Well, the most important characteristic of 4th generation district energy system is low temperature supply water. As buildings become increasingly energy-efficient, it will be possible to lower the temperature of the water significantly – without compromising the comfort of the consumer. Today’s supply water is typically 90°C degrees, but to help reduce heat losses, temperature may be reduced to as little as 50°C in 4th generation systems. The same goes for district cooling – only here the temperature will be raised from approx. 6° to as much as 12° in flow temperature.

This new take on water temperatures has some obvious benefits:
- First of all, less energy is needed to heat or chill the water, reducing costs and CO2 emissions.
- Secondly, less energy is lost in the distribution network through the pipes as thermal losses.
- And thirdly, the pipework will suffer far less thermal stress, which prevents leakages and increases the lifetime of the system.

There is, however, another important benefit: Low temperature supply water in District Heating enables the district energy plant to increase its share of renewable energy sources and include low-temperature sources like geothermal energy. It also increases the efficiency of solar thermal collectors, wind energy, and surplus heat from industrial processes. This way, plants can draw on a mix of locally accessible sources to support a sustainable production.

4th generation district energy plants will also have the ability to store surplus heat over long periods of time. Seasonal storage in large, heavily insulated underground pits allows the plants to harvest, for instance, solar energy during the summer and use it to heat homes during the winter. This utilisation of otherwise wasted energy is an important aspect of improving sustainability in the sector.

While all these perspectives offer great opportunities to reduce our global energy consumption, there are some challenges to overcome. For instance, when you reduce the temperature of the supply water, you get a lower delta T. Due to the lower delta T, you’re risking that consumers at the furthest end of the network won’t achieve the minimum differential pressure unless plants increase pressure on the supply side.

However, this challenge can be overcome by applying the principles of pressure distribution: Instead of relying on one powerful supply pump at the plant, pressure distribution employs a range of pumps on the way to the consumer. In doing this, the pumps can run at much lower speed, saving energy. By using variable speed pumps to supply smaller sections of the grid, the system is also able to adjust to actual demands instead of running at full speed. Running at full speed can cause reduced pressure in the network, thereby increasing the risk of leakages.

So to sum up:
4th generation district energy supplies heated water at a lower temperature and chilled water at higher temperature to energy-efficient buildings. This decreases energy usage and energy waste, while increasing system lifetime and fuel flexibility. Renewable energy sources, seasonal storage and strategic use of variable speed pumps all add to the fact that 4th generation district heating is a viable solution to the great energy challenges of the 21st century.