by Linda Bertelsen
97percent renewable

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 SOfrom 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.

Illustration by Ole B. Winther og Sofie Malm
Illustration by Ole B. Winther og Sofie Malm

Another challenge Geoclima and Unicool faced was the size of the equipment room where the heat pumps are installed – like many district heating plants, it’s particularly narrow. This required Geoclima to design a dedicated pipeline and layout, to enable easy maintenance, modular usage, and quick parts replacement. Unlike traditional technologies, which are received in parts, assembled onsite, and filled with refrigerant, the new electric heat pumps, because they are built and charged at the factory, took less than a week to install, start, and commission.

Meanwhile, the compressor’s compact size made it easier to achieve the required 2.7m x 1.2m footprint for the heat pumps 2 and 3. The variable high-speed centrifugal compression enables heat pumps to be smaller, as oil-free compressors like the TTH and TGH have roughly 30% of the cubic footprint of an equivalent capacity screw compressor. Additionally, the other  components developed and optimized for oil-free applications and ultra-low-GWP refrigerant HFO1234ze helped ensure reliable and efficient Geoclima heat pump operation.

How oil-free technology is heralding a new era of district heating

Although relatively new to the district heating heat pump market, the oil-free technology – like the TTH and TGH models used at Ringsted – are an increasingly popular choice, because they offer a wealth of benefits that traditional oiled screw compressors can’t match:

  • Low maintenance: The oil-free action enables the design of a simplified system, reducing the time and cost of maintenance significantly.
  • Consistent high performance: Two-stage oil-free magnetic bearing-based centrifugal compression, and built-in variable speed, ensure extremely efficient, oil-free operation. As no oil is used to lubricate the compression process or bearing system, there’s no chance of oil fouling within the heat exchangers. And with contact-free operation, there’s no metal-to-metal contact degradation. This means there’s zero performance degradation over time – and no efficiency loss.

Oil-free compressors are also small and quiet – making them perfect for district heating pants or any other location where noise levels are a concern. Magnetic bearings and centrifugal compression ensure that the compressor works at extremely low noise levels – on average, around 8dB(A) lower than an equivalent capacity screw compressor.

The medium- and high- lift compressor models used in the Ringsted DHC heat pumps also ensure no direct global warming potential, as they utilize the ultra low-GWP HFO refrigerant R1234ze. And the hermetic factory pre-packaged heat pump design helps ensure the refrigerant can’t leak into the environment.

The future of district heating: are you ready?

Efficient district heating shouldn’t impact the environment – and with oil-free heat pumps, it doesn’t have to.

Introducing electric water-to-water heat pumps, and other heat recovery components allowed Ringsted DHC to capture every last kilowatt of surplus heat – and use it to boost the district heating system capacity and efficiency. Not only does this allow the plant to hit key decarbonization goals, minimize dependence on fossil fuels, and improve plant efficiency, it also minimizes the price of heat for local residents.

More importantly, it highlights the opportunities oil-free heat pumps can offer to other district utility providers and municipalities. And shows how, as environmental regulations get stricter, you can meet them head-on.

For further information, please contact: Drew Turner, drew.turner@danfoss.com

Watch the Video: Heat from Exhaus Gas Scrubber

Meet the author

Drew Turner
Director of Global Business Development and Market Research, Danfoss
97% renewable – Ringsted DHC’s heat recovery kickstarts a new era of greener district heating” was published in Hot Cool, edition no. 1/2021. You can download the article here: