Cooling Demand Calculation Analogous to Cooling Degree Days

The degree-hour model can be applied not only to assess heating demand but also to determine cooling demand. As with the heating model, the base temperature is compared to the outdoor temperature to calculate the cooling demand using available data. For each hour, the necessary thermal energy is computed to maintain the desired indoor temperature. This building model is particularly suitable when detailed building data is unavailable. Polysun also offers alternative approaches to incorporate cooling energy demand into the design of energy systems.

Entering Energy Demand

To calculate the cooling demand using the degree-hour model, building consumption data is required. Polysun offers three different input options for this purpose:

Annual energy demand/losses

The annual cooling demand is specified in kWh. This model is suitable, for example, when the cooling demand is known or has been calculated elsewhere.

Monthly energy demand/losses

As for the annual energy demand, this definition method allows an energy amount in kWh to be specified, but the input must be provided separately for each month. This approach is particularly suitable for buildings whose cooling demand deviates from the typical seasonal distribution.

Maximum power demand

For this selection, the maximum power demand must be entered.

The maximum possible internal heat gains are calculated based on user-defined specifications:

<p><p><p><p><p><p dir=”auto”>\(Q_{gain,max} = Q_{dem,max} \cdot c_{gains}\)

Here, \(Q_{dem,max}\) is the defined maximum power demand of the building, and \(c_{gains}\) is the gain calculation coefficient.

In the next step, the UA value is calculated, taking into account the maximum outdoor temperature and the set indoor temperature:

\(UA = \frac {Q_{gain,max}} {T_{amb,max} – T_{set}}\)

From this, the heat gains are calculated, but only for outdoor temperatures that exceed the target indoor temperature:

\(Q_{gain}[h] = \frac{UA \cdot (T_{amb} [h] – T_{set})} {1000}\)

\(Q_{gain} = \sum Q_{gain}[h]\)

The cooling load is then calculated from the heat gains and the gain calculation coefficient:

\(Q_{dem,cooling} = \frac{Q_{gain}}{c_{gain}}\)

Once the method has been selected and the necessary information entered, Polysun will obtain the required building data. The simulation can then be run.

The results can be displayed at various resolutions, including monthly and hourly values, as well as timestep values.

Calculating Cooling Energy Demand: Methods and UA Value Differences

For all types of energy demand input, the cooling energy demand and energy gains are either directly queried in the building’s context menu or calculated from user inputs (e.g., maximum power demand). The method for calculating the cooling energy demand is analogous to that for the heating energy demand, with the formulas described in detail.

Cooling vs. Heating Demand Calculation: UA Value Calculation Based on Heat Gains and Losses

The cooling demand is calculated based on the building’s heat gains. Heating demand, in contrast, is determined by heat losses. An example below illustrates how the UA value calculation for cooling energy demand differs from that for heating energy demand.

Cooling load: \(UA_b = \frac{Q_{gain} * 1000} {Sum_{T_{diff}}}\)

Heating load: \(UA_b = \frac{Q_{loss} * 1000} {Sum_{T_{diff}}}\)

Selection of Heat Gains

The cooling energy demand can be influenced by the selection of heat gains. If the heat gains are set to “high,” the user can define the base temperature themselves. If the “normal” setting is chosen, Polysun calculates the base temperature, just as it does for the heating model.

Energy Deficit

The energy deficit is calculated as the difference between the cooling energy demand and the cooling provided by the convector:

\(Q_{def} = Q_{dem,cooling} – Q_{conv}\)

A warning appears if the calculated energy demand is not met and one or both of the following scenarios occurs:

• The energy deficit exceeds 50% of the energy demand for a total of 12 hours in at least 10 consecutive periods

or

• The energy deficit exceeds 30% of the energy demand for a total of 24 hours in at least 5 consecutive periods