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Heat pump

A heat pump raises heat from a low to a usable temperature level. How much electricity it needs for this is not a device constant but depends on the operating point – on the temperature of the heat source and on the required flow temperature. This is why EPOS-Plan does not calculate with a blanket seasonal performance factor but with the complete manufacturer performance map.

Coefficient of performance and seasonal performance factor

The theoretical limit follows from the Carnot process: the smaller the temperature lift between source and sink, the higher the achievable coefficient of performance. Real machines reach about 40 to 50 per cent of it. As a rule of thumb every degree of higher flow temperature costs roughly two and a half per cent – between 35 and 55 °C this amounts to about a third in efficiency. The coefficient of performance describes one operating point, the seasonal performance factor the whole year. In EPOS-Plan the latter is therefore a result and not an input: it follows from the hourly balance and comes out lower the more often the machine has to work at low source and high flow temperatures.

Performance map and interpolation

For every flow temperature the map holds heat output and coefficient of performance for each source temperature. In the simulation the values are interpolated linearly between the supporting points for every hour, so output and power draw match the actual operating point. If the temperature falls below the stored range, the program asks whether to extrapolate – a deliberate stop, because extrapolation beyond the tested range quickly becomes unrealistic. Performance maps can be taken from the catalogue, created manually or imported according to VDI 3805.

Heat sources

Ambient air is the easiest source to develop but delivers the worst temperatures exactly when demand is highest. Ground and groundwater offer almost constant source temperatures and therefore more stable performance. Waste heat from processes, exhaust air or waste water is often at a high level but available only to a limited extent and usually only during operating hours. EPOS-Plan covers all of this through the selectable heat source: ambient air from the climate data, a constant source temperature, a buffer tank as source with limited content and adjustable regeneration, a user-defined source profile from monthly and weekly values, or an hourly profile imported as CSV.

Flow temperature and heat sink

The flow temperature is the most important lever of efficiency and is determined by the heating system – underfloor heating, existing radiators or hot water preparation. Because hot water requires a considerably higher level than space heating, a heat sink can be assigned per machine in EPOS-Plan: total demand, hot water only or space heating only. This makes separate generators for the two tasks realistically modellable instead of running a single machine permanently at the higher level.

Bivalence, back-up heater and lock-out periods

A heat pump is rarely sized for the design heat load of the coldest day. Common practice is to size it for part of the load with a second generator for the few cold days: in alternative operation the second generator takes over completely below the bivalence point, in parallel operation both work together, in partially parallel operation the heat pump switches off below a defined temperature. EPOS-Plan does not prescribe the bivalence point but determines it as the highest ambient temperature at which uncovered demand still occurs, and reports the share of the electric back-up heater – a share above roughly five per cent indicates an undersized machine or too high a flow temperature. Utility lock-out periods are entered as time windows and visibly shift the bivalence point.

Operating modes

Runtime-optimised runs the machine at maximum output and fills the store – long runtimes, few cycles. Output-optimised delivers exactly the heat currently required – lower storage losses, more modulation. PV-optimised runs preferentially when solar power is available and uses buffer tank and building mass as thermal storage; this often raises self-consumption more effectively than additional battery capacity. With several machines the priority determines the order of use.

Concept with a booster heat pump

The conflict between efficient heating and hygienic hot water can be resolved hydraulically. In the booster concept the central heat pump delivers heat at a low level with 35 to 45 °C flow temperature into a buffer tank and thus stays permanently in its efficient range. A small booster heat pump uses this buffer as its source at 30 to 40 °C – an unusually good source temperature – and raises the temperature to 60 to 65 °C to charge a hot water buffer. A fresh water station heats the drinking water instantaneously, so no drinking water is stored and the legionella risk is avoided by design.

The gain lies in the seasonal performance factor of the main machine, which no longer has to reach high temperatures for hot water, and in the clear hydraulic separation of the two tasks. Against this stand the second refrigerant circuit, additional investment and maintenance. Whether it pays off depends above all on the hot water share: the higher it is – apartment buildings, hotels, care homes – the clearer the advantage. In EPOS-Plan the concept is modelled with two heat pumps with different heat sinks, each with its own flow temperature, performance map and priority; the booster machine is given the buffer tank as its heat source. The corresponding schematics are shown under Fundamentals.

Sizing in practice

Calculate variants instead of using rules of thumb: the same machine at 55 and at 40 °C flow temperature, with and without a buffer tank, with and without a booster. Four figures are meaningful – the back-up heater share, the determined bivalence point, the full-load hours and the minimum peak boiler output required. For planning heat pump systems in residential buildings, VDI 4645 is the relevant German guideline; the evidence it requires corresponds exactly to the inputs EPOS-Plan asks for.