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# Heat loss in heat storages

How large are heat losses in thermal storages? On this page you find a compact guide to estimate heat losses for thermal storages.

## What does affect heat losses?

The losses of a heat storage depend primarily on the so-called A/V ratio, i.e. the ratio of storage surface (A) to storage volume (V). The storage surface determines how large the losses are (double the surface leads to double the heat loss). The volume determines the energy content of the storage tank. For very large storage tanks, the A/V ratio is very small and the heat losses relative to the energy content are also small. In addition, heat losses depend strongly on the thickness of thermal insulation. This cannot be estimated across the board and depends on the manufacturer of the storage tank. In summary, the storage losses depend on:

• Ratio of storage surface to storage volume (A/V)
• Temperature of the medium in the storage
• Thickness of thermal insulation
• Ambient temperature
• Temperature stratification in the storage

## How large are the storage losses in heat storages?

In the nPro tool, the losses of a thermal storage can be defined by specifying the relative loss per time, e.g. "1 % per day". This means that the storage tank content decreases by 1 % over one day. Thus, if the storage is fully charged, the loss is higher than if the storage is almost completely discharged. This makes sense because a storage tank at a high temperature has higher (absolute) storage losses than a storage tank close to ambient temperature. In Table 1 rough estimates are given, which can serve as a first orientation, if no further information about the storage is available. The data are based on own calculations and literature data. However, since numerous assumptions had to be made, e.g. on the storage tank temperature or insulation thickness, the data is only a rough guideline. For small storage tanks with up to about 1 m³, the estimate "10 % per day" can be assumed. For seasonal storages, the figure "35 % per 365 days" can be found in the literature.

Table 1: Heat losses depending on storage size (orientation values)
Storage volume Relative heat loss
0,75 m³ 10 % per day
30 m³ 20 % per 5 days
300 m³ 10 % per 10 days
3000 m³ 15 % per 30 days
70000 m³ 35 % per 365 days
In the nPro tool standby heat losses are given in the form "X % per Y days".

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