4th generation district heating and cooling in buildings In large complex buildings, e.g., hospitals, the designers often look for decentral solutions as an alternative to the traditional centralized ventilation systems, e.g., to install a large number of decentralized ventilation systems with a local source for heat- ing, cooling, and dehumidification. In such cases, a 4th gener- ation district heating and cooling system inside the building is the perfect solution. Each ventilation unit will be connected to the building-level DHC system and equipped with a coil for cooling and a coil for heating. If the building can be connected to a city-level DHC system, the pipes can continue directly into the building, and each de- centralized unit will, in principle, be a DHC consumer. In case city-level DHC is unavailable, a building-level heat pump con- nected to a ground source system (ATES) and even with a heat storage tank and a chilled water tank can be almost as efficient as a large DHC system. It can later be an integrated part of a larger DHC system. In case odd end-use demand - which deviates from the demand for comfort, e.g., refrigeration or process heat, and therefore cannot be served directly by the DHC system - a local compressor can upgrade the cold or heat to the re- quested temperature and use the DH as a cold sink and the DC as a heat sink. This is, e.g., best practice at universities, e.g., at DTU, and it is demonstrated on a large scale by Høje Taastrup District Heating in Greater Copenhagen serving 70 end-users inside a large building owned by Copenhagen Markets, a whole market for the sale of fruit and vegetables. A compressor boosts the temperature from 10 °C in the dis- trict cooling system to minus 8 °C in the antifreeze network to the building. The CO2 negative 4th generation DHC To be independent of fossil fuels, we must produce renewable fuels in e-fuel factories combining CO2 from Carbon Capture (CC) and H2 from electrolysis, as electricity cannot supply all end-use demand. CO2 will suddenly be a resource. CO2 from waste incinerators, biomass CHP plants, and plants for upgrading biogas to biomethane are the most cost-effective sources of CO2. The surplus heat from this generation can be utilized by 4th generation DH. In a few years, a new infrastructure for CO2 will connect the largest point sources of CO2 with facilities for e-fuel production, storage, and shipment. H2 from electrolysis can act in the electricity market like heat pumps, as they can interrupt at any time if needed in case of a shortage of renewable electricity (albeit with a small stand-by electricity consumption). Moreover, electrol- ysis can offer upregulation services shifting the power con- sumption from maximal demand to zero. And finally, the process generates surplus heat, which can be used as DH
– some of it directly without heat pumps. A new hydrogen infrastructure will supplement the methane gas grids and interconnect electrolysis with large industrial consumers and fuel factories. Finally, the fuel factories producing electro fuels combining CO2 and H2 can offer surplus heat for the DH, and the elec- tro fuels can be stored and used by the DH to replace the fossil fuels in the back-up boilers. Good cases In Denmark and other countries benefitting from DH, the virtu- al battery is utilized by more than 100 district heating compa- nies, mainly in Denmark, Sweden, and Finland. This operation is not only profitable for the district heating consumers but also important for the electricity system. You may find several good cases in the reports from JRC, the Joint Research Center of the EU, and the State of Green. The CCU (Carbon Capture and Utilization) and CCS (Carbon Capture and Storage) are not fully commercial. Still, the first full-scale projects are in the pipeline in Denmark as well as plans for a CO2 infrastructure, e.g., the C4 collaboration (Car- bon Capture Cluster Copenhagen). Electrolysis is also not fully commercial, but projects are in the pipeline both on a small and large scale. Considering that the losses can be reduced significantly if the heat is utilized, it is likely that such plants can be among the main sources of DH in larger cities. It is essential to remember energy planning at a national and local level, as the challenge will be to plan and develop a resil- ient, low-carbon infrastructure most cost-effectively. This is the main message in the UN SDGs (United Nations Sustainable Development Goals), and the EU enforces it in the Energy Effi- ciency, Renewables, and Buildings directives. Virtual battery The district heating systems, which use electricity for heating can do it in a very smart way taking into account the fluctuations of the prices. Large electric boilers use large capacities of electricity at low prices and pre- vent curtailment of wind and solar. Large electric heat pumps generate most of the heat, but can be interrupt- ed as long time as needed at high prices. CHP plants can take over and generate electricity at high electricity pric- es. Heat storage tanks accumulates the cheapest heat for later use and minimize the use of back-up capacity from boilers. Seen from the power system, it looks like there is installed a huge electric battery in the district heating system.
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