By Anders N. Andersen, Head of Energy Systems Department, EMD International A/S Poul Alberg Østergaard, Professor in Energy Planning, Aalborg University
The changing role is important to keep in mind when designing DHCP plants – while at the same time keeping in mind that the ongoing electrification of society requires that heating and cooling production at DHCP stations primarily will be served by electrical heat pumps using e.g. surplus, low-grade heat from industry, sewage systems or seawater. In Denmark, 16 central power plants providing district heating to big cities, 285 distributed CHPs providing heat for towns and villages and 380 industrial and private CHP plants proving heat for private and public DH grids have played an important role in providing an efficient energy system. The installed electrical capacity at the central power plants is 5,693 MW, 1,887 MW at the distributed CHP-plants and 574 MW at industrial and private CHP plants – however, they are currently threatened by decreasing spot market prices. This article argues that distributed DHCP CHP - representing 23 % of the total electrical capacity in Denmark - is quickly moving to the capacity-providing phase – perhaps even too quickly! As shown in figure 1, the electricity production at the distributed DHCP CHPs has been decreasing rapidly in recent years as a natural consequence of wind power developments. Wind power has come to cover a major part of the electricity demand, leaving less electricity to be produced by CHP. Until 2005 the Danish DHCP CHPs was paid under a fixed tariff system thus producing independent of wind production, but from year 2000 the wind turbines started to be curtailed from time to time - amplified by the situation that DHCP CHPs continued to produce because they were operating under the fixed tariff system. This also indicates the point in time where the energy system role of CHP changed from being a provider of electricity produced efficiently in cogeneration mode to being a provider of flexibility. As a consequence of the changed role, from 2005 to 2015, the fixed tariff system was phased out for progressively smaller DHCP CHP sizes, turning these over to be market-operated and most of these being traded on the Scandinavian day-ahead market.
District Heating and Cooling Plants (DHCP) will have both the opportunity to provide efficiency and flexibility through e.g. CHP, heat storages and heat pumps – while at the same time providing capacity when required. Fortunately, through investment decisions made a long time ago, Danish DHCP plants are already partially equipped for this role but to take on the role as capacity provider of the future, further incentives have to be established. Combined heat and power (CHP) units at DHCP have an important, but changing, role to play in the transition to a renewable energy system. Their initial role is merely to displace fossil fuelled condensing mode power generation, however, the development in Denmark shows that this role changed to providing flexibility for accommodating fluctuating renewable energy sources instead. Also, in the future, the role will be to provide electrical capacity during the hours where fluctuating renewable energy sources are unavailable. In the first phase, the aim is simply to cover the heat demand to the highest extent possible by DHCP CHP plants thus providing electricity where all coproduced heat is utilised. Generally, for each MWh of power thus replaced on a condensing mode power station by DHCP CHP, the same quantity of fuel is saved, often called the CHP benefit. With an increasing penetration of fluctuating renewable energy sources – mainly wind, solar, and wave power – it becomes increasingly important that the DHCP CHPs act with more flexibility and assist in the integration of these. One of the consequences is a decrease in the power generation from DHCP CHP in the next phase. As the energy systems progress towards fully renewable energy systems, based on fluctuating renewable energy sources, very little production is left on DHCP CHP and they have only to produce electricity in the relative few hours where fluctuating renewables are insufficient. At this stage, it is questionable whether there is any surviving condensing mode capacity, however multi-purpose DHCP CHPs may still present a business case, amongst others providing needed electrical capacity.
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