An energy efficiency of more than 100%

Green transition 6 November 2017 Claus Hindsgaul Tore Hulgaard

At Amager Bakke waste-to-energy facility in Copenhagen, a net energy efficiency of 107% is expected. When thermally treating mixed municipal solid waste, securing an energy efficiency of this size is a genuine possibility.

12 min

Written in collaboration with Tore Hulgaard. Originally published in Ingeniøren (in Danish).

In order to grasp how an energy efficiency of more than 100% can be achieved, it is important to understand that the efficiency of energy plants in Europe is traditionally and almost always calculated on the basis of the Lower Heating Value (LHV) of the fuel. This applies to everything  from natural gas boilers, biogas engines and wood chip fired power plants to waste-to-energy plants. 

The LHV is the energy release by combustion defined as the potential energy recovery when the flue gases from the fuel are cooled to 25°C without condensation of generated water vapour. The definition is therefore based on the assumption that the condensation heat from water vapour in the flue gas is lost. 

At Amager Bakke the flue gases are cooled to around 25°C in the flue gas condenser of the plant, allowing almost 100% recovery of the energy expressed by the LHV even if the water vapour had not condensed. The total energy recovery exceeds 100% when the heat of condensation is released as a consequence of the inevitable condensation of water vapour at this temperature. 

Even though the energy efficiency calculated using common methods exceeds 100%, more than 100% of the theoretical energy content of the fuel, expressed as the Higher Heating Value (HHV), is naturally not recovered. The HHV is defined as the potential energy recovery when cooling the flue gases to 25°C including condensation of water vapour. This actually corresponds precisely to what will be done at large scale at Amager Bakke. 

When the facility’s own consumption of electricity (approx. 3% of the energy content) and modest process losses are subtracted, Amager Bakke will generally recover 95% of the HHV for electricity and district heating. 

Mixed, moist waste with an energy content (LHV) of 11.5 GJ/t has a corresponding HHV of around 13 GJ/t. A waste-to-energy plant without losses would then be able to reach an energy efficiency on mixed waste of 13/11.5=113%. Amager Bakke will be able to reach 95% of this, which is 107%. 

No waste fractions take away energy from the waste-to-energy facility – neither the wet fractions, nor the ones that cannot be combusted. Even “the wet tomato” makes a positive contribution with its energy content. The water content of the waste evaporates and condenses at the facility without having any negative impact on the total efficiency. 

The content of metal, glass, ceramics and stone in the waste does not, as one might otherwise assume, lead to any significant loss of energy despite it being heated in the furnace to around 500ºC as part of the bottom ash. The reason is that the bottom ash is cooled in a water bath and the vapour from here is eventually condensed and the cooling energy recovered in the flue gas condensation. 

Furthermore, the handling of bottom ash and other residues involves little use of energy. The metal from the waste is separated from the bottom ash and recycled, while the rest of the bottom ash matures and is applied for construction works. No energy intensive processes are involved apart from electricity for conveyors and sorting systems and energy for other mechanical handling. 

The total energy consumption is therefore less than 1% of the energy produced at the facility and
of minor significance in a total perspective. 

Finally, it should be noted that modern waste-to-energy facilities are established for flexible energy supply purposes. Amager Bakke will be equipped with Denmark’s largest heat pump installation and has like other facilities the option of operating on a varying load. 

In this way the production can to a very large extent adjust to the heat demand, and like other facilities Amager Bakke can adjust the power production to the market and contribute to a stabilisation of the power grid.