By combining heat recovery and absorption heat pump technology with CHP plants, total system efficiency can be increased beyond 100%, explains Jens-Ole Aagaard Jensen, citing two district heating schemes in Denmark as examples.

Wood chip boiler on a heating plant of 8.5 MW capacity

Old technology, used in new ways can create unique results, and this is the case with a new idea from the Danish company Hollensen Energy, which develops solutions for heating and boiler manufacturing ranging from individual boiler systems to turnkey solutions for heat and power plants.

It is no secret that one of CHP technology’s greatest challenges is that return water temperature is often too high. The many causes include how the distribution grid is operated and poor consumer understanding of how to use district heating.

Hollensen Energy has long worked to remedy or at least minimize this waste of energy – and, by examining new and established knowledge, its technicians and engineers have now found a solution. By adding an absorption pump to the CHP cycle to recycle flue gas for a second time, an increase in the overall system efficiency of up to 112% has been achieved.

By combining two known technologies – CHP and absorption heat pumps – Hollensen has significantly raised the usefulness of both.


Hollensen Energy has already installed absorption pumps at two plants that supply two small communities in Denmark. In both Langa and Bjerringbro virtually all households are connected to the district heating network.

These district heating systems already had very high operating efficiencies of about 95–98% – due largely to a good heating network and Danish consumers’ familiarity with using district heating – with return water temperature at slightly more than 40°C.

But total efficiency has been improved to between 106% and 112%, and even bigger gains are available when new technology is placed where the return water has a higher temperature and even more energy is wasted.

Often the total system efficiency is as low as about 80%. This figure can, with a single heat pump and a modification of the circuit where flue gas is recycled a second time, be increased significantly.

In principle, exactly the same technique can be applied to a compressor pump, but this requires expensive energy in the form of electricity for producing cooling water.


Since the first plant with an added absorption pump delivered the expected results, the company experimented further, using the same technology with different fuels.

With biomass, total utility efficiency can hit 125–130%. This fits well with the company’s desire to supply ’energy with care’. Indeed, Hollensen Energy is far from completing its study and development of the technology.

The company sees considerable potential for adding absorption pump technology to CHP in other applications, both large and small plants. The device can benefit all firms that produce heat – both heating only and CHP – and firms that sell heat.

In principle, the system works independently of grid network efficiency and the technology can still achieve a high utility efficiency irrespective of the return water temperature.

Hollensen Energy has more than 30 years’ experience in heating and boiler manufacturing and its staff can be involved all the way from drawing board to construction site. Installations and plants are designed in continuous dialogue with the customer, so that the company can provide an optimal solution, followed with effective service after delivery.Hollensen Energy works with gas- and oil-fired boiler plants, gas-fired power plants and biomass-fired boilers using wood chips, straw, waste wood or wood pellets. The firm has worked on plants ranging from 400 kW to 15 MW per unit.

Jens-Ole Aagaard Jensen is the CEO of Email:

Absorption heat pump technology

An absorption heat pump applies the two principles that water boils at different temperatures depending on pressure, and that a lithium bromide-water (LiBr–H2O) solution is water absorbent (hygroscopic). An absorption heat pump has four main components:

  • generator – which supplies energy for the heat pump;
  • condenser – which condenses super heated water vapour;
  • absorber – which absorbs refrigerant under heat;
  • evaporator – where the LiBr solution evaporates and cools the system water.

To operate the plant, heat must be injected into the generator. This energy comes from the flue gas from the wood chip boiler. The cooling process is started by adding heat to the generator. This evaporates the water in a LiBr–H2O solution that flows to the condenser. In another circuit the remaining part of the solution (now low in water) flows from the generator to the absorber. The vaporized refrigerant from the generator condenses in the condenser and is carried to the evaporator. In the evaporator, the refrigerant is sprayed across a pipe section in which the system water that supplies the cooling capacity flows.

By this means the system water is cooled down while the temperature of the coolant increases. The heating causes the refrigerant to evaporate. From the evaporator the vaporous refrigerant is fed to the absorber and the refrigerant is absorbed by the solution. This releases heat to the heating circuit.

The solution, now enriched with refrigerant is pumped through a heat exchanger and back to the generator. The circuit is now complete and the process starts again.

View of the flue gas system on a wood chip-fuelled heating plant

Enhanced district heating installations

Billund CHP

Numbers of inhabitants: 6200 within city limits

Numbers of meters: 2200 units

Total installed capacity: 41 MW thermal

Number of gas engines: 5 units for a total of 10.9 MW electricity and 14 MW thermal

Number of gas boilers: 5 units for a total of 20 MW thermal

Wood chip: 1 unit with condensing and absorption pump, totalling 6.9 MW thermal with efficiency 118–122%

Grid: 47 km main pipe and 51 km service pipe

Grid loss: 17–19%

Temperatures: forward 82°C and return 40°C (the system is flow controlled)

Bjerringbro CHP

Numbers of inhabitants: 6800 within city limits

Numbers of meters: 2250 units

Total installed capacity: 47 MW thermal

Number of gas engines: 4 units for a total of 12.8 MW electricity and 18 MW thermal

Number of gas boilers: 3 units for a total of 29 MW thermal

Grid: 51 km main pipe and 40 km service pipe

Grid loss: 15.5%

Temperatures: forward 69°C and return 39°C

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