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Accumulation Tank for Heat Storage

Accumulation tanks store “cheap” RE-produced heat from systems like solar heating plants, heat pumps, or electric boilers (powered by cheap electricity during off-peak hours) and from biomass plants, which operate best continuously without frequent starts/stops. They are also used to store engine heat from natural gas-driven CHP plants.

The capacity of an accumulation tank depends on its size, the temperature of the heat, and heat loss. It requires a pipe connection to the plant’s other production facilities and should ideally be located nearby.

With an accumulation tank, it is possible to balance varying heat production from RE plants like solar heating systems and electrically driven production units. Operating these plants at low electricity prices can be economically beneficial when the electricity market needs to create quick consumption to downregulate or balance the grid capacity. An accumulation tank can store heat produced at low electricity prices for later use.

Technology

An accumulation tank is usually a vertical welded insulated steel tank constructed on-site. Its size depends on the amount of energy to be stored, the temperature conditions within it, and the storage needs relative to the connected production plants.

Tank volume and heat loss in accummulation tanks

Figure 1: Tank volume as a function of stored energy amount concerning the temperature difference in the tank.
Figure 2: Heat loss in the accumulation tank as a function of tank volume indicated by different insulation thicknesses.

Source:Guide to Fossil-Free Peak Load in District Heating Systems”, p. 36-38. Danish Energy Agency (in Danish)

The accumulation tank contains heat with a temperature of 95-98°C at the top. It distributes downward in layers, so the bottom temperature often corresponds to the return temperature from the district heating network. Charging and discharging the tank occurs at the top, where the heat is either stored or extracted, as this is where the temperature is highest.

The space requirement for an accumulation tank can be adjusted within certain limits, as it is possible to reduce the diameter by increasing the height and vice versa. However, changes in geometry can affect the layering within the tank, potentially resulting in a more mixed forward temperature. Additionally, the tank can be used to maintain static pressure in the system if it is an open tank, which again requires a minimum height.

The placement of the accumulation tank should facilitate easy connection to existing district heating production facilities for straightforward heat accumulation. It is an environmentally green solution with no waste products, noise, or odor.

Accumulation tanks are not suitable for long-term storage, such as seasonal storage.

Operation and Maintenance

An accumulation tank has virtually no or very low operating costs. A pressurized tank will have costs related to operating the pressure maintenance system (nitrogen plant). Additionally, depending on the operating mode and water quality, a diver/rover may need to inspect the tank internally for potential damage every few years.

Economy

Investing in an accumulation tank allows for storing energy for later use. Besides storing heat for peak and reserve load needs, an accumulation tank can contribute to lower heat prices because it can store heat produced with cheap electricity in the electricity market.

As mentioned under operation and maintenance, operating costs are limited to external tank maintenance and internal inspections with a diver or rover every few years. Increased heat loss in the system should be expected, which must be added to production costs.

System Integration

An accumulation tank is primarily used as a storage unit to level peak loads. It can also be used as a reserve load with limited capacity. The capacity depends on the size of the storage unit in the given situation.

Some plants actively use accumulation concerning maintenance periods on their base load units, increasing production from base load units before the maintenance period to charge the tank and then discharging from the accumulation tank during the maintenance.

Depending on the heat demand in the network, heat can be stored for a few days in some cases.

Regulatory Processing

Establishing an accumulation tank requires a project proposal and an EIA (Environmental Impact Assessment) screening. Local plan conditions and possibly the municipal plan must be examined, particularly regarding height constraints that may limit the size of the accumulation tank.

A building permit must be applied for construction.

Establishment

Choosing and establishing an accumulation tank for peak/reserve load depends on several factors, including the need to store heat from existing or upcoming production plants and the suitability of the production units for storage. The need for extra capacity in a peak/reserve load situation, which an accumulation tank can provide, will also depend on fuel prices, especially if the tank is intended to exploit fluctuations in electricity markets. This factor complicates determining and calculating the profitability of an accumulation tank.

Profitability depends on the number of operating hours with cheap electricity available at a price that makes it advantageous to produce heat beyond the actual need. This must be weighed against the overall need for peak reserve load, which an accumulation tank can contribute to supply security.

Economic and Technical Data for an Accumulation Tank

DescriptionValueUnit
Heat capacity*)35/50MWh per 1,000 m³ volume
Investment cost135-335€/m³
Fuel typeSolar, biomass, electricity, and CHP (natural gas)
Efficiency98 (including heat loss)%
Variable costs~0€/MWh
Fixed costs **)Diver inspection ~ 2,000-2,700€/dive
Expected construction period ***)6-9Months
Area requirement ****)~200-300 m² – height=10-25 m
Lifespan20Years
Functional capabilities
Peak/reserve load, storage of electricity from the spot market, regulation power  
Table 1

Table 1: Economic and technical data for an accumulation tank.
*) Top temperature 70/95°C and bottom 40°C.
**) Supplier information.
***) Experience data from completed projects, excluding project proposals, regulatory processing, and grid connection.
****) Experience data from completed projects.

Source: Guide | Fossil-Free Peak Load in District Heating Systems. The guide is prepared for the Danish Energy Agency by Grøn Energi in collaboration with the district heating sector. The guide originates from the Climate Agreement of June 22, 2020, and its measures to support green district heating initiatives.

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