How to size a heat storage system

How to calculate heat-store size, related to heat demand and heat production, and also assess the benefit of increasing the flow/return differential temperature.

Required hot-water-storage volume depends on what amount of heat needs to be stored over the highest demand cycle period (normally a day) and the temperature between the flow and return pipes to and from the store.

The amount of heat storage for a water-based system is given by the formula:

Q = V × (Tf - Tr ) × 1.162

Where:

Q is the amount of heat stored in kWh

V is the volume of water in the tank in m³

Tf is the flow temperature from the store in °C

Tr is the return temperature to the store in °C

1.162 is derived from the specific heat capacity of water (4,180 J/kg°C) divided by time (3,600 seconds).

By rearranging this equation, the volume of hot water needed to store a given amount of heat (Q) is given by:

For example, if heat required by one hectare of glass during the day is 10,000 kWh and the flow and return temperatures of the store are 85°C and 55°C respectively, then the heat-store size to deliver all of this heat would be:

Interestingly, the sizing ‘rule of thumb’ for heat storage for a salad crop is 200 m³/ha.

Clearly, it is important to take into consideration the highest heat demand during the period when waste heat is available. Also, note the flow and return temperatures from the store are important, as the closer they are together, the more storage is needed to deliver the same amount of heat.

When considering heat-store requirement to buffer a biomass boiler, it is necessary to take into account the amount of buffering time that is required. For example, you may wish to keep a biomass boiler running for, say, two hours, even when the heat demand has disappeared, just to stop the boiler from unnecessary and inefficient cycling.

Take a 1 MW boiler (1,000 kW) which needs a two-hour buffer for efficient operation. Heat storage is therefore: 2 hours x 1,000 kW, which is 2,000 kWh.

Using the equation above, and based on the same flow and return temperatures, the volume of storage required would be:

The rule of thumb for biomass boiler heat stores is between 50 m³/MW and 100 m³/MW.

The principles shown here can be used to determine store size, related to heat demand and heat production, and also assess the benefit of increasing the flow/return differential temperature.