The city energy company Helen is building hot-water storage into man-made caves
Former fuel-oil storage caves in the Helsinki bedrock will store hot water heated in the processes of the Helsinki energy company Helen, to be used for district heating from 2021 onwards. The hot-water lakes will be the newest energy storage facility in Helsinki and complement underground cold-water lakes used to cool the city.
The man-made caves, now being converted from oil to water storage, are located in the Mustikkamaa recreational island of Helsinki. The caves will be filled with 260,000m3 (70 million gallons) of domestic water, which will be kept at maximum 90 °C (194 °F) through a district heating connection from Helen’s heat production processes to the island.
The heat stored in the water will be released into the district heating network with heat exchangers. The full energy storage capacity of the caves – 120 MW – will be enough to provide for half of Helsinki’s summer-time district heat consumption (hot domestic water) for 4 days. It can be used in winter to ease peak demand, which would otherwise necessitate extra generation in coal- and oil-fired processes.
“Because heat doesn’t have to be produced as it is consumed, heat energy storage will compensate for energy production with fossil fuels. It is one step in our shift away from fossil fuels,” says Rauno Tolonen, Helen’s climate and energy efficiency manager.
“The significance of heat storage will be growing as we shift more and more to renewable energy. The shift will entail higher fluctuation in the availability and cost of energy,” Tolonen explains.
The underground hot-water lakes of Mustikkamaa will be similar to underground lakes of cold water used for cooling in Helsinki’s district cooling network.
Two caves built into the Helsinki bedrock – one under the downtown Esplanade Park for 25,000m3 (6.5 million gallons) and the other in Pasila for 11,000m3 (3 million gallons) of cold water – are integrated with heat pumps, which work with the cooling network to produce district heat by recycling energy.
As cold water circulates in the network releasing its cooling energy, it recovers heat from properties. Heat pumps recover the heat and feed it into the district heating network.
A new heat pump was added to the Esplanade facility in summer 2018, which increased the share of district heat produced with heat pumps in Helsinki to 10%. Heat pumps are used to recover heat from diverse available sources, including treated wastewater and data centers.
Heat pumps now produce more than 90% of Helsinki’s district cooling, which is Europe’s third largest and fastest growing network.
Helen has plans for more energy storage, investigating possibilities to build the world’s first seasonal heat storage facility into former fuel-oil caves located in the residential development area of Kruunuvuorenranta. The caves would be filled in summer with seawater at around 18 °C (64 °F) and used in winter as a heat source for heat pumps to produce district heating water at over 80 °C (176°F). Heat recovered from local properties would be an additional heat source.
Energy storage and heat pumps are elements of Helen’s extensive palette of solutions to shift to climate-neutral energy production. Helen seeks to cut carbon dioxide emissions fully by 2050. Helen’s interim goal is to cut the use of coal by 50% and carbon dioxide emissions by 40% from 1990 to 2025. Today 90% of Helsinki’s heated building stock uses Helen-supplied district heat, 61% of which is produced with coal and 28% with natural gas at Helen’s four combined heat and power (CHP) plants in Helsinki.
Helen’s plan for climate neutrality vitally supports a plan of the City of Helsinki to render Helsinki carbon neutral by 2035. The City of Helsinki has prepared a detailed action plan outlining how to reach the target. An interim target for Helsinki is to cut greenhouse gas emissions by 60% from 1990 to 2030.
Helen Ltd is a fully owned energy company of the City of Helsinki.