97% RENEWABLE – RINGSTED DHC’S HEAT RECOVERY KICKSTARTS A NEW ERA OF GREENER DISTRICT HEATING

Ringsted District Heating Company (DHC), a large district heating utility in Denmark, has reduced its reliance on fossil fuels by 97% after installing an innovative heat recovery system using heat pumps. The heat pumps – built with oil-free compressor technology – recover surplus heat from the plant’s equipment that would otherwise be wasted.

By Drew Turner, Director of Global Business Development and Market Research, Danfoss

This is a significant step forward for the utility and district heat suppliers everywhere looking to provide more efficient, environmentally friendly heat.

Traditionally, district heating networks have relied on fossil fuel-sourced heat generation to supply consumers with comfort space and water heating. But with regulations and growing costs, district heat plants are turning to renewable energy and innovative technology like electric heat pumps and heat recovery to reduce their environmental impact and give consumers an affordable price.

This article will examine Ringsted DHC’s new system and how implementing new heat pump technology has enabled the station to hit critical heat efficiency and decarbonization goals. We’ll also explore how oil-free technology is paving the way for greener, more efficient district heating.

Keeping Ringsted warm – and hitting decarbonization goals

Ringsted DHC delivers heat to the city of Ringsted across a 124km district heating network, with most of the heat supplied from one centralized heating station. With the equivalent of 7,000 one-family homes to keep warm, heat must be transmitted safely, without interruption, and at the best price for the consumer.

Before last year, the station generated its heat using two straw-fuelled biomass boilers, a gas-powered Combined Heating and Power (CHP) plant, and a heat accumulator. This meant that 75% of its heat was generated from renewable sources.

However, the DHC had also promised the city that its heat production would be 95% CO2-free by 2020. Moreover, the exhaust gas from its straw-fired boilers contained harmful sulfur dioxide – as did the oxidization catalyst used in its gas-driven CHP. Nearly all (97%) of this needed to be removed.

It was a significant challenge. But by making smart use of heat pumps to ensure no surplus heat goes to waste, Ringsted DHC has been able to meet its goals while keeping its consumer heat prices low.

Three new heat pumps

In 2020, the DHC introduced four new electric heat pumps – three of which use Turbocor® oil-free technology – and a new scrubber to remove SO₂  from the straw boilers’ flue gas.

The new pumps help to capture what would otherwise be wasted heat from its existing infrastructure and the outside air and put it to good use – increasing COP, maximizing heat capacity, and further reducing its environmental impact.

The first heat pump (HP1) recovers heat from the outside air, then heats up the district’s returning flow to 57°C. This is mixed with water from the biomass boilers at 95°C and sent back via the district heating network.

But this was just the beginning. Ringsted DHC realized that the flue gas from the straw boiler was a significant source of wasted heat and that this needed to be cooled significantly from its escape temperature of 120-140°C for the scrubber to be effective. So, it introduced a scrubber and two Geoclima heat pumps (HP02 and HP03) with a total 1000kW heat capacity, powered by Danfoss Turbocor oil-free compressor technology.

These new heat pumps perform an important double job – recovering energy for the district while also eliminating the need to provide the scrubber with an expensive external water supply. As a result, the overall system is efficient and cost-effective.  

Finally, the DHC wanted to recover heat from the new heat plant equipment room, the CHP, and the variable speed drives for the ambient air-sourced heat pumps (HP01). So it introduced an additional 350kW heat pump (HP03), using oil-free compressors, to recover a further 310kW from its equipment, put it to use for the city, and further improve the plant efficiency.

Higher heat recovery temperature and lower heat supply temperature result in higher heat capacity & efficiency

To get the highest heat pump efficiency and lower the price of heat, Ringsted DHC needs to recover the heat at the highest temperature possible.

The HP02 heat pumps operate at a high heat recovery temperature (51°C> 28°C surplus heat from the straw boiler scrubber), resulting in a high heat capacity of 962kW and a COP of 7.1. HP03, which uses waste heat from HP01, the CHP, and from cooling the heat plant room, also runs at a high heat recovery temperature and, as a result, has a heat capacity of 310kW and COP of 6.2. The final supply heat temperature is 58C, which is low enough to enable the high efficiency of all three units.

HP01, which is challenged by operation with heat recovery from the lower ambient air temperature during the winter months and air-to-refrigerant heat exchanger approach temperature while also supplying a higher heat capacity of 6.829-9.500kW at a corresponding COP of 3.0-4.1 (depending on outside air temperature).

The heat recovery methodology utilized for the Geoclima heat pumps enables improvement of the overall heat plant COP and heat capacity (up to 21% and 31%, respectively). Maintaining a balanced temperature and cooling with resulting recovered heat has also helped make the plant’s equipment more reliable.

The unique challenges of installing heat pumps at Ringsted DHC

For Geoclima and Unicool, who designed and delivered the heat pump units for Ringsted DHC, one of the biggest challenges was optimizing pumps to handle highly variable temperatures.

Condensers had to work from 50°C to 67°C, while the evaporator was handling temperatures from 35°C to 21°C – a big step up from the 6/7°C evaporator cooling temperatures compressors work with for comfort cooling. Geoclima also needed to ensure temperature control at various floating water flows, which could drop from 100% at full load to just 10%.

With any vapor compressor, the design must be optimized to the conditions in which it will operate. Ringsted DHC’s HP02 and HP03 required two different optimized compressor designs: a medium- and high-lift version. The medium-lift version is the optimal efficiency solution for HP02 units, consistently supplying a heat forward condenser temperature of 57°C. HP03, which is required to supply 67°C hot water temperature periodically, incorporates the higher lift-optimized compressor design.