Estimating the building demand

One of the key input parameters for any geothermal simulation is the building demand for heating and cooling. Sometimes, these values are known based on detailed simulations or measurements, but more often—particularly in the early stages—they need to be estimated. This article sheds some light on the topic and explores several approaches to addressing this challenge.

Thermal building demand in GHEtool

Within GHEtool, it is possible to input various types of load profiles. First, a distinction is made between hourly and monthly resolutions, with 8,760 and 12 time steps respectively. Typically, the time scale of the data covers a single year, meaning it is assumed that the building demand remains the same each year throughout the entire simulation period. For phased projects, where the building demand evolves over time, you can also use data with a multi-year time scale. This article focuses on the former, while another article will explore multi-year data in more detail.

Hourly resolution

Designing a borefield using an hourly resolution will provide the most accurate results, as peak powers (and their durations, as discussed in this article) should no longer be estimated. Hourly data is typically generated from dynamic building simulations (using software such as IESVE, DesignBuilder, IDA ICE, etc.), or in the case of a renovation, may be based on measurement data.

!Note
It is important to note that a higher resolution does not automatically guarantee more accurate results. If the hourly profile is not representative of the actual building demand, the outcome may be misleading. In such cases, a well-estimated monthly profile may yield better results. However, when reliable hourly data is available, it remains the most accurate approach.

!Caution
Hourly demand profiles are sometimes extrapolated from other projects. This can result in inaccurate designs, as peak powers and durations are highly sensitive to building characteristics. Ensure that the demand profile you are extrapolating from is truly representative of the building being designed (for example, extrapolating from one apartment within the same apartment block).

Monthly resolution

When no hourly data is available, a monthly simulation can be performed. To do this, four key inputs are required—five if domestic hot water (DHW) demand is included: the peak power for both heating and cooling, as well as the annual energy demand for heating and cooling. Each of these is discussed in more detail below.

!Note
Although we refer to a monthly resolution, you can simply enter the annual values in GHEtool Cloud, which will automatically distribute them across the months—saving you considerable time.

Peak power for heating

The peak power for heating corresponds to the peak output of the building’s heat pump. If the heat pump has a capacity of 10 kW, the building cannot draw more than 10 kW, and this value should therefore be set as the heating peak. Depending on regional building standards, this heat pump capacity is typically based on a static heat loss calculation relative to a reference outdoor temperature (e.g. –8 °C in Belgium).

Modulating heat pumps

Let’s say the calculated static heat loss of a building is 7 kW. An installer might choose to install an 8 kW heat pump to allow for some oversizing. However, this would lead to an oversized borefield, since the borefield must be able to cope with the peak demand. If the system uses a modulating heat pump, you can often set a lower peak power in the heat pump controller—for example, limiting the 8 kW heat pump to operate at a maximum of 7 kW—thus avoiding unnecessary oversizing of the borefield. The same reasoning applies if the emission system (e.g. radiators or underfloor heating) cannot utilise the full heat pump capacity.

!Caution
Even if the building’s static heat loss is below the heat pump’s rated capacity, a modulating heat pump may still operate at full power, depending on the internal control logic. Therefore, if your borefield is sized for a power lower than the heat pump’s maximum output, it is essential to ensure that the control settings of the heat pump enforce this power limit.

Early stage

If the heat pump specifications are not yet known, peak heating demand can be estimated using established rules of thumb. These are based on either the capacity of the emission system or general guidance related to building type and age. A sample table with typical values is provided below, though be aware that these figures are region-specific and should be adapted to your local context.

(Data obtained from the Cooling 2.0 project)

(Data obtained from nPro, values for the climate of Berlin)

Simultaneity

When multiple heat pumps are connected to a central, shared borefield, the resulting peak power is not simply the sum of the individual units. Instead, a simultaneity factor must be applied. This was discussed in one of our previous articles, which you can find here.

Peak power for cooling

The approach to defining the peak cooling power depends on whether the borefield is actively designed for cooling or whether cooling is considered a secondary benefit, a ‘nice to have’.

Design for cooling

In warmer climates, where cooling plays a more significant role than heating, the borefield is typically limited by the peak injection temperature (see our article on borefield quadrants for more detail). In such cases, the cooling demand is usually determined in accordance with local building regulations, taking into account factors such as glazing area, g-value, and U-value. Based on these calculations, the building’s cooling demand is derived in a similar manner to the heating demand.

Nice to have

In regions where cooling is not a primary concern, the emission system is generally not sized to guarantee thermal comfort in summer. Here, cooling is often treated as a “nice to have” feature—an added benefit of installing a geothermal borefield. In this case, the peak cooling power is usually based on the capacity of the emission system (originally designed for heating). Refer to the table of typical emission system capacities provided earlier.

Early stage

In the early stages of a project, the peak cooling demand—like the heating demand—can be estimated using basic rules of thumb, drawing on either the emission system’s capacity or values from similar reference buildings.

Energy demand for heating

While the peak power is one important aspect of a building’s energy profile, the other key parameter for borefield design is the annual energy demand for heating. This can be estimated in several ways: using full load hours, rules of thumb, or heating degree days.

Full load hours

Using full load hours is a straightforward method to estimate the annual energy demand based on the known (or estimated) peak power. If a system operates at peak load for x number of hours per year, the total energy demand is given by: $$energy = peak \cdot FLH$$. Below is a table with indicative full load hour values for various building types.

!Caution
As discussed previously, the installed heat pump may be oversized relative to the building’s actual peak demand. To avoid overestimation, it is advised to apply the FLH to the static heat loss of the building rather than to the installed heat pump capacity.

(Data obtained from SenterNovem, Cijfers en Tabellen 2007)

Rule of thumb

Just like with peak power, annual heating demand can also be estimated using empirical values based on floor area. The table below gives typical heat demand values for buildings in the Berlin climate region.

(Data obtained from nPro)

Heating degree days

Another method to estimate heating demand is by using Heating Degree Days (HDD). HDDs quantify how much (and for how long) the outside air temperature is below a certain base temperature—referred to as the balance point temperature, below which the building requires heating. The total HDD is the sum of daily temperature differences between this base and the actual outside temperature, over a heating season.

HDD-based methods offer a more refined estimate compared to FLH because they account separately for building characteristics (insulation, solar gain, etc.) and climate conditions.

!Note
The balance point is not simply the thermostat’s setpoint. In the EU, it is typically taken as 15.5°C, accounting for internal gains, building mass, and other factors.

Energy demand for cooling

The same principles that apply to heating also hold for cooling. When no detailed hourly data is available, the cooling demand can be estimated using either full load hours (FLH) or rules of thumb.

Full load hours

For Belgium’s climate, typical values for FLH in cooling range between 500 and 1000 hours. Once the peak cooling power is known or estimated, the energy demand can be calculated as:

$$energy = peak \cdot FLH$$

Rule of thumb

Alternatively, you can use benchmark values for annual cooling demand per square metre. The table below provides such values based.

(Data obtained from the Heat Roadmap Europe)

Cooling degree days

Just as Heating Degree Days (HDD) can be used to estimate heating demand, Cooling Degree Days (CDD) offer a way to estimate cooling requirements. CDD are calculated based on the difference between a balance point temperature (commonly 18°C) and the actual outdoor temperature, whenever the latter exceeds the balance point.

Two empirical formulas—developed by the European Commission (ENER/C1/2018-493, doi: 10.2833/158083).

For space cooling in the residential sector:

$$FLH=96+0.85\cdot CDD$$

For space cooling in the tertiary sector:

$$FLH=475+0.49\cdot CDD$$

Energy demand for DHW

An increasingly important parameter in borefield design is the demand for domestic hot water. In most residential contexts, a typical value is around: 1000 kWh/person/year. However, this value can be significantly higher in buildings like hotels and hospitals.

!Note
One challenge when estimating the domestic hot water (DHW) demand lies in deciding whether to base it on the current number of residents or the maximum potential occupancy. Imagine, for example, a building with three two-person bedrooms, currently occupied by only two elderly residents. Should you size the borefield based on just 2 occupants (i.e. 2 MWh/year), or on the full capacity of 6 people?

If the aim is for the geothermal borefield to serve as a long-term asset, it should be capable of accommodating the building’s full potential, not just its current usage. Therefore, we recommend estimating the DHW demand based on the maximum reasonable number of residents, rather than solely on the current occupancy. This ensures the system remains future-proof and fit for purpose, even as occupancy changes.

Conclusion

This article discussed the thermal demand of buildings and how it can be estimated. A distinction was made between building demand data with an hourly and a monthly resolution — with the former offering the most accurate results, although it is often unavailable. For the monthly resolution, various guidelines and rules of thumb were presented to estimate the building’s energy demand for heating, cooling, and domestic hot water.

References

• Watch our video explanation over on our YouTube page by clicking here.
• The emission power of different emission systems was obtained here.
• The load profiles of nPro can be found here.