Termisk energilagring i fjärrvärmenät

Sammanfattning: District heating is the most common form of heating in Sweden and as other industries it faces challenges in terms of increased resource utilization and transition to 100% renewable energy sources. Thermal energy storage in district heating networks can bring several advantages, for example enabling the use of excess energy, reduction of equipment size and capacity, as well as load leveling. By replacing peak load equipment, thermal energy storages can be an important enabler for systems entirely based on renewable energy. In district heating networks today, buffer tanks are usually used to meet short-term variations, other storage solutions are rare. This study aims to contribute with knowledge about the potential of alternative thermal energy storages in the district heating networks. This is done by compiling comparable parameters for commercialized storages and storages in development phase. The parameters used within the study are energy density [MWh/m3], power density [kW/m3], investment cost per installed capacity [kSEK/MWh] and power [kSEK/kW], charging temperature, discharging temperature, and return temperature during charging [°C], efficiency [%], system integration, operational availability, implementation and environmental impact/emissions. Among the sensible storages the identified techniques are pressurized buffer tanks, aquifer-, borehole-, rock cavern-, pit- and hot water storages. For latent storages, which are in the development phase, experiments, pilot plants and studies have been compiled. The same applies to thermochemical storages. For comparison with a fully commercialized technology, data for buffer tanks has been compiled. Thermochemical storages based on sorption have the highest energy density followed by hot water storages, latent storages with inorganic phase change materials, pit storages and thermochemical storages based on chemical reactions without sorption. Pressurized buffer tanks have the highest power density followed by latent storages with inorganic phase change materials, buffer tanks and thermochemical storages based on sorption. The investment cost per installed capacity is significantly lower among sensible storages than among latent and thermochemical storages. Among the sensible storages, the investment cost is lowest for borehole, aquifer and hot water storages. For latent storages, the investment cost per installed capacity differs markedly depending on whether the phase change material is organic or inorganic where the storages with organic phase change material presents higher investment costs. The investment cost of the thermochemical storages varies but is generally lower than the cost of latent and higher than the cost of sensible storages. The investment cost per installed power is lowest for buffer tanks followed by other sensible storages with water as storage material (pressurized buffer tank, pit storage, rock cavern storage and hot water storage). The remaining storages has several times (75 - 216) higher investment costs per installed power. Efficiency for pressurized buffer tanks, hot water storages, latent storages and pit storage is highest where all have maximum values above 80%. Aquifer and borehole storages have efficiencies around 50%. For rock caverns the reported value for efficiency is 65%. Information about efficiency is missing for thermochemical storages. The temperature levels differ between the various installations, but in summary, the temperatures for the Swedish aquifer storages are relatively low with discharge temperatures between 8 - 28 ° C. The discharge temperatures of the borehole storages are between 35 - 55 ° C. For storages with water as storage material (rock caverns, pits and water tanks), the temperatures are generally higher. The upper limits for the working temperatures in pit and hot water storage is 95 and 90 ° C respectively. The discharge temperatures for rock caverns vary between 65 and 82 ° C.  With the exception of some high temperature storages, the reported working temperature of latent storages is up to 90 ° C and the discharge temperatures for thermochemical storages 30 - 120 ° C.  The sensible storages are implemented centrally in their systems. Among the latent and thermochemical storages, the majority are placed centrally in their systems, but storages close to the property and mobile storages occur. At current investment cost, economically competitive alternatives to buffer tanks are questionable for storages where high effects are required. Borehole-, pit- and rock cavern storages are considered to have the potential to fit as long-term storage in district heating networks as they have relatively low investment costs per installed capacity and high storage capacities. Pit- and rock cavern storages could also be suitable for meeting short term variations as the storage medium, water, can theoretically reach high discharge power.