Increased energy optimisation using low temperature convectors

A new method of low-temperature heating system makes it possible to reduce energy use by at least 30% and shortens the payoff time for housing cooperatives. The system isn’t technologically challenging and is a cost-effective way to better transfer heat and cold

The heating of apartment buildings accounts for a very large part of Sweden's energy use. The requirements for profitability in energy renovations, low energy prices in Sweden, and limited opportunities for increased rents are some reasons for the low energy renovation rate.

The rapidly increasing demand for energy-efficient cooling alternatives makes it interesting to look at an interconnected system. With the new method, it’s possible to install a new heating system and get cooling “for free”. This is possible thanks to heating systems with low temperatures and free-cooling with high temperatures which enables energy optimisation both at the property level and at the infrastructure level.

When comparing the pay-off time of the low exergy heating- and cooling system to constructing a conventional cooling system, the payoff time will be 0 years compared to 15 years.

The technical solution is obtainable by replacing conventional radiators with low-temperature convectors, which also enable free cooling from boreholes, the threshold for energy renovation can be lowered. The lower system temperature can provide benefits in the form of reduced electricity for heat pumps, lower return temperature in district heating networks and free cooling. The system can also maintain a constant and low flow, which facilitates stable regulation during the different seasons. In general, about 30% lower energy use is achieved, due to reduced over temperatures, with a well-functioning adjustment [1].

The method needs collaboration between different system aspects:

• Instead of a conventional radiator, a high-efficiency heat exchanger was used. Its compact dimensions made it suitable as a convector.
• Conventional radiators emit most of their heat through natural convection. The prototype heat exchanger requires forced convection by a fan but achieves a very low-pressure drop.
• Conventional radiators receive significantly higher heat dissipation if it preheats incoming supply air, due to a large temperature difference between the air and the heater. Especially in winter, the cold supply of air will increase the heat dissipation from the radiator.
  • The prototype is heating incoming supply air (supply air diffuser on the back), and the room-heater are serial-connected. Since increased temperature difference and the forced convection of the supply air make the heat transfer [2] more efficient, it will be possible to achieve a high heat power even with the low return temperature from the room heater.

The greater the airflow, the more efficiently the surrounding surfaces on walls and ceilings are heated, which increases its heat radiation. Increased operating temperature contributes to increased comfort [2]. This is made possible by a combination of a highly efficient VVX, forced convection and serial connection with the return temperature to a supply air heater provides high efficiency and compact dimensions.

Thanks to the system's favourable temperature it’s possible to get a heat pump to work more efficiently, with less electricity.

Martin Larsson, RISE (E-mail)

• Heating
• Cooling
• Case studies

References

[1] Svensk Fjärrvärme (2003). LÅNGTIDSEGENSKAPER HOS LÅGFLÖDESINJUSTERADE RADIATORSYSTEM. FOU 2003:88

[2] Högskolan Dalarna. (2000). LÅGTEMPERATURVÄRMESYSTEM -En kunskapsöversikt. EKOS PUBLIKATION 2000:4