Heat pump performance

The coefficient of performance (COP) and capacity (CAP) of GSHPs are highly influenced by the entering fluid temperature (EFT). GHE Analysis supports four input modes for heat pump performance, ranging from a simple constant value to a full temperature dependent COP and CAP.

Temperature limits (T_Lim) - These fields allow you to specify the entering fluid temperature beyond which the operation of the GSHP is not recommended or even impossible. In the current version of GHE Analysis, no control measure stops the GSHP if EFT exceeds T_Lim. It is therefore the user's responsibility to modify their design in order to maintain the fluid temperature within a range suitable for the selected heat pump. T_Lim is also used by the Sizing tool to determine the required borehole field length.

Degradation coefficient (C_d) - Part-load operation of a GSHP can lead to on-off cycles that degrade thermal performance. GHE Analysis accounts for this through a degradation coefficient (C_d) applied to the average part-load ratio (PLR) over each hour. A C_d value of 0 corresponds to zero degradation in heat pump performance. A value of 0.25 is recommended for most GSHPs (Fuentes et al., 2016).

DHW COP - When DHW loads are defined in the building loads panel (Hourly Separate C/H + DHW or Monthly mode), two additional fields appear: the DHW COP in cooling and the DHW COP in heating.

These values set the electricity used for domestic hot water. How much of the demand is covered at each hour follows the rules given on the Building loads page:

• In cooling, at a DHW COP of 5, about four-fifths of each unit of hot-water energy comes from recovered condenser heat and one-fifth from electricity.
• In heating, at a DHW COP of 3, about two-thirds of each unit comes from the ground and one-third from electricity.
• When the building load is zero, or during geocooling hours, DHW coverage is zero.

The default cooling DHW COP is ∞, the top of the field's range: the hot water then costs no electricity, all of it coming from condenser heat that would otherwise be rejected to the ground. Enter a finite value when the tank must be heated above the condensing temperature by a booster; the lower the value, the larger the share of electricity.

The electricity used for hot water is added to the system total and has its own Hot water sCOP line on page 3 of the console. These fields have no effect in the Hourly Combined or Ground load modes, and they are disabled when a geocooling system type is selected.

Input mode - Four input modes are available from the drop-down list:

• Constant performance — A single COP and CAP pair is specified for cooling and heating. Performance is assumed independent of EFT. This mode is appropriate for preliminary sizing when no manufacturer data is available.
• Performance vs. temperature — COP and CAP are entered directly as a function of EFT in the table (columns: EFT, CCAP, CCOP, HCAP, HCOP). Rows can be added with the button, removed with the button, pasted from the clipboard (Ctrl+V), or imported from a CSV or tab-delimited text file with the Import button. GHE Analysis fits a smooth, shape-preserving (PCHIP) curve through the entered points.
• Manufacturer data — Raw data from a manufacturer's datasheet is entered in the table (columns: EFT, TC, HR, HC, HE, where TC is total cooling capacity, HR is heat rejected, HC is heating capacity, and HE is heat extracted). GHE Analysis automatically derives the cooling and heating COP from these values. Cells containing zero appear in orange to indicate missing data.

Summary & charts: COP and capacity curves versus entering fluid temperature

The two charts display the COP (top) and CAP (bottom) curves as a function of EFT. The solid lines are PCHIP interpolants of the entered data; the solid circles are the data points themselves. The vertical dashed lines mark T_Lim for cooling (blue) and heating (red). Small dots scattered around the curves represent part-load operating points; they are invisible when C_d = 0 or the heat pump operates at full load, as they then coincide with the manufacturer's curve.

The summary below the charts reports the COP and CAP of the GSHP at T_Lim for each operating mode. The total installed capacity equals the number of heat pumps (n_HP) multiplied by the individual capacity at T_Lim, i.e. n_HP × CAP(T_Lim).

GHE in practice: performance sensitivity to EFT, cycling degradation and model selection

The COP of a GSHP varies significantly with EFT. A 5 °C rise in EFT during cooling, or a 5 °C drop during heating, typically reduces the COP by 10–15 %. Oversizing the GHE to limit EFT excursions therefore directly improves system efficiency. The degradation coefficient C_d captures the additional performance penalty from cycling, which can be significant in buildings with low part-load fractions. Using manufacturer data rather than constant performance values leads to more reliable energy predictions and avoids underestimating the electrical consumption of the GSHP system.

The choice of temperature limits has a large influence on field sizing: for a heating-dominant building — one whose annual ground load in heating exceeds that in cooling, so the field length is governed by the coldest EFT reached in winter — a 1 °C change in the minimum EFT limit in heating mode can produce a 10–15 % change in total borehole field length. Setting T_Lim,H too high forces a significantly larger field; setting it too low risks allowing the EFT to drop below the heat pump's safe operating range during peak heating conditions. When setting T_Lim,H, maintain a safety margin of at least 5–6 °C above the freeze point of the heat carrier fluid. This margin is necessary because the fluid temperature inside the heat pump evaporator can fall locally below the leaving fluid temperature (LFT) at the heat pump outlet. The Fluid Type module computes the freeze point for the selected fluid and concentration; consult it when finalising T_Lim,H.

In practice, choose the input mode that matches the data you can defend: a manufacturer datasheet for a final design, and a constant COP only for a first screening. Keep in mind that the entering fluid temperature the heat pump sees is governed by the distribution side as much as by the ground — lower heating supply temperatures and higher cooling supply temperatures both ease the duty on the loop and lift the seasonal COP. Selecting equipment that performs well across the EFT range your field will actually produce is often more valuable than chasing a high rated COP measured at standard conditions.

A higher-COP heat pump typically costs more; in heating it also draws more heat from the ground per unit of energy delivered to the building, which increases the annual ground load and therefore the required borehole field length and its capital cost. Chasing a high rated COP to achieve operating savings can therefore trigger capital cost increases on both the equipment and the geothermal field that more than outweigh the savings. Life-cycle cost analysis - not rated COP alone - should drive the equipment selection decision.