Thermal storage sizing algorithms

The Hysopt optimiser offers three ways to determine the volume size:

1. Manual

2. Peak shaving and

3. Anti-cycling.

Manual

The manual option allows the user to define the size of the thermal storage/buffer as in the previous software versions.
In addition, you can use the Hysopt’s sensitivity tool to do automated ensitivity analysis on the storage tank volume, as described in Sensitivity Analysis

Peak shaving (when using DHW)

Peak shaving is the process of using a buffer vessel to limit the size of the production units placed. By placing a larger buffer vessel, a smaller production unit can be used, having positive effects on CAPEX costs. Before the calculation method was added, you needed to do simulation manually in order to properly calculate the needed size, now however, the peak shaving algorithm will determine this for you.

The commonly used algorithm depends on 3 parameters, which become available when clicking the button to edit “Peak shaving”.

• – time to fully charge the storage/buffer tank,

• – duration of the peak of DHW demand, and

• – the storage/buffer efficiency.

With this information, the minimum volume size is calculated by using the following relationship:

where:

– additional power to heat up the buffer tank,

 – primary (related to HIUs) system flow temperature,

– primary return temperature in the heating mode, and

– specific heat of water.

The additional power () is given by:

where:

 – the primary power during the DHW peak demand and

 – primary power demand in heating mode.

Finally, the volume of the buffer is defined as

Anti-Cycling

Anti-cycling algorithm is based on the Dutch ISSO 96 standard for the dimensioning of a buffer tank in connection with a CHP (Combined Heat and Power) system. The determination depends on:

• preventing the CHP unit from too often starting (anti-cycling function); the active system content is too small for the unit to meet a small heat demand and

• to cover peaks in heat demand and to allow the CHP unit to operate at other times.

The buffer volume is calculated as:

where:

  – buffer volume, [],

  – cycle time, [h],

  – density, [],

  − specific heat of water, [],

  – temperature difference between upper and lower buffer temperature, [^oC],

  – minimum power of CHP, [kW] and

  – minimum heat load, [kW].

When the user presses the button to edit “Anti-cycling”, the following parameters can be selected from the pop-up menu:

  – minimum production power,

  – minimum heat demand,

  – cycle time,

  – flow temperature,

 – return temperature,

 – storage efficiency.

Based on the parameters defined above, the volume storage is determined by Eq.(1).