The thermal stores in Hysopt can be used for central heating (CH) or domestic hot water (DHW).

Central heating storage units

Direct, indirect and electrical

The central heating storage units can be divided into three types: Direct, Indirect, and Electrical.

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The direct units don’t have a coil functioning as a heat exchanger, separating the primary side with the secondary side, while the indirect units do. The hydraulic separation is in most cases needed when glycol is used on one side and not on the other.

The electrical storage units have an electric resistance embedded into the thermal store and therefore also function as a “production” unit, besides being a thermal store.

Besides the visualized base circuits (BCs) above, more BCs are available combining different types or allowing multiple connection possibilities.

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Two-pipe and three-pipe storage tanks

In addition to thermal storage tanks with four (or more) pipe connections, the Hysopt library also includes two buffer vessels (BCs) with respectively three and two pipe connections (see figure below). All variants in Hysopt provide hydraulic separation, which is why the two- and three-pipe tanks feature wide connections at both the top and bottom.

Depending on the designer’s preferences and the specific application, two- or three-pipe storage tanks can offer certain advantages. For example, these configurations allow supply water from the primary side to be delivered directly to the secondary side without passing through the tank. This is particularly useful during start-up, as it ensures the secondary side receives sufficiently high flow temperatures without the need to partially charge the buffer.

The three-pipe variant includes a direct connection at the top for supply, while the bottom features two connections. This setup ensures that the return water from the secondary side passes through the buffer before returning to the production units, which helps to mitigate highly fluctuating temperature swings in the return water to more stabilised return temperatures towards the primary side.

Additional variants

Furthermore, there are 3 more exotic thermal storage units.

The first one is a thermal store in the return pipe, which is only used in specific circumstances (mostly used for cooling but on rare occasions also for heating).

The second one is a thermal store with multiple temperature measurements. This BC is mostly used as a simplified thermal store representing multiple thermal stores in series, where for instance 1 thermal store has 3 temperature measurement points, the second thermal store only 2 points, and so forth. The multiple measure points are often used in large hybrid systems where the priority of each type of production unit is decided in the controls and the applied measure points.

The third one is a thermal store in the supply pipe. In contrast to all other stores, this one only has a capacitance as design parameter. Its main purpose is to put thermal mass into the system.

Design parameters

The parameters needed for the design of the system are mostly the design heat flow and the temperature regime.

Both parameters must be locked in order to apply the filled in values. If not, the Optimiser will propagate both the design heat flow and temperature regime from the end-unit side (secondary side) through the thermal store to the production side (primary side).

The design heat flow can be changed and locked by the user. When doing so, the temperature regime is propagated through the thermal store but the primary thermal power is overwritten.

The user can also lock the temperature regime (or both the regime and thermal power), resulting in overwritten primary design temperatures.

Remark: For multiple primary and/or secondary temperatures and thermal powers, the secondary design conditions can’t simply be propagated to the primary side since the distribution should be specified in the thermal store. In this case, the design conditions of these BCs with multiple connections on either side should be manually entered by the user. Otherwise the default values are used, which are in most cases incorrect.

Besides the thermal power and regime temperatures, the design volume of the thermal store is needed for the total fluid amount calculation. The user should enter this manually because this isn’t calculated automatically in the software. If you need to calculate the buffer vessel sizing for a CHP of HP you can find this in this file.

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The thermal stores with a coil as a heat exchanger (indirect units) also have the KV value, which should be entered by the user to accurately calculate the pump head on the primary side.

If the actual KV value isn't known, the user can click on the "pencil" icon which results in a calculation popup window. In the popup window, the user can calculate the KV value by entering the pressure drop and flow rate (and if needed, the brine, mixture, and reference temperature). This option is preferred when the information on the boiler is known. However, the user can also let the software automatically calculate the KV value depending on the design flow rate by using the other tab and entering the estimated pressure drop over the boiler.

Simulation parameters

The parameters needed for simulation are the following:

The environment temperature is needed for simulating the thermal losses of the thermal store. The measured cells in percentage are needed for specifying the height of the temperature measurement points. The user can change the values in order to adjust the control behaviour of the thermal store to their needs.

The thermal store volume is overwritten by the design volume entered by the user and is used with the diameter in simulation as a 10 cell capacity model. The 10 cell capacity model divides the thermal store vertically into 10 cells. Each cell is a small cylinder with a specific temperature variation to accurately simulate the stratification. All the cells are stacked on top of each other and transfer energy to each other and the environment.

The diameter is needed to specify the height and weight of the thermal store and each cell. The U-value is also needed to calculate the thermal losses to the environment and represents the insulation grade of the thermal store. By default, the U-value is set to 0.4 W/m²K.

For each connection side, the height of the supply and return connection can be altered by the user. For instance, when a pipe is connected to the middle of the thermal store, the user can simply enter 50%.

Remark: When the thermal store has only 1 connection at the top and/or 1 at the bottom, the user can enter 100% for the supply connection (on both sides) and 0% for the return connection (also on both sides). This way the visualization shows 4 connections going to the thermal store, but in simulation, it is accounted for only 2 connections. The single pipes towards the top and/or bottom of the thermal store are however not dimensioned and included in the simulation.

The thermal stores with a coil as a heat exchanger (indirect units) also have the UA value which is needed to simulate the thermal power transfer from the primary side to the thermal store itself.

Remark: For units with electrical resistance, the design heat flow is needed for simulation not for design since there is no primary side to propagate to. The design heat flow is used in simulation to specify the amount of thermal power going into the thermal store. Seeing as it is a production unit as well, a control signal is needed to activate and/or modulate the thermal power delivered by the electric resistance.

Domestic hot water

Domestic hot water store units are similar to the central heating store units, except that they no longer have a secondary gate. Instead, a tap profile from the control library must be attached in order to generate a load towards the heating store. More information on a tap profile can be found here: Tap profile generators As there is no longer a circuit downstream of the BC, the design heat flow & temperature regime from the BC’s parameters are propagated further upstream, even if those values aren’t locked.