Coefficient of performance heat pumps

If we try to assess the relative economy of a heat pump versus burning fossil fuel directly, we must bear in mind that, if the electrical energy to run the heat pump comes from fossil fuel, the power plant is subject to the Carnot limitation. The overall efficiency of a modern steam power plant is about 35 percent. Thus, just to break even on fossil fuel consumption, the heat pump coefficient of performance must be at least 1/0.35 = 2.9. 

The Carnot efficiency is always smaller than unity, so the Carnot heat pump coefficient of performance is always greater than unity. The amount of heat delivered to the hot reservoir is always greater than the work put into the heat pump because some heat has been transferred from the cold reservoir to the hot reservoir. There is no violation of the Clausius statement of the second law because the heat pump is driven by another engine. A real heat pump must have a lower coefficient of performance than a reversible heat pump but can easily have a coefficient of performance greater than unity. 

The efficiency of a heat pump is measured by the coefficient of performance, COP ... 

We examine the performance aspects of the system heat pumps by considering three types of coefficient of performance Carnot, actual and exergetic. 

The energy (or first law) efficiency is simply a ratio of useful output energy to input energy and is referred to as a coefficient of performance (COP) for refrigeration systems. The energy efficiency of the heat pump unit can thus be defined as follows ... 

Here, COPCamot is the maximum heating coefficient of performance, based on a Carnot (ideal) heat pump system operating between low- and high-temperature reservoirs at Tl and Th, respectively. 

Figure 16.38 shows a heat pump appropriately integrated against a process. Figure 16.38a shows the overall balance. Figure 16.38b illustrates how the grand composite curve can be used to size the heat pump. How the heat pump performs determines its coefficient of performance. The coefficient of performance for a heat pump can generally be defined as the useful energy delivered to the process divided by the power expended to produce this useful energy. From Figure 16.38a ...

Tong Y., Kozai T., Nishioka N., Ohyama K. (2010). Greenhouse heating using heat pumps with a high coefficient of performance (COP). Biosystems Engineering, 106(4), 405-411. 

Itis sometimes convenient to define a coefficient of performance for a refrigerator as p = 2i/W, which is equal to (1/e) — 1. Similarly, a coefficient of performance for a heat pump can be defined as 7 = — Q-ilW, which is equal to 1/e. Thus, a reversible Mctionless heat pump transfers as heat to the high temperature reservoir a multiple of the work done on the engine, and, in some circumstances, a real heat pump can be more economical than direct combustion as a source of heat. 

The COP (coefficient of performance) of the Carnot heat pump (or refrigerator) operating between a fixed high-temperature thermal reservoir at Tfi and a fixed low-temperature thermal reservoir at Tl is irrespective of the working substance. 

Coefficient of performance of a refrigerator Php Coefficient of performance of a heat pump A Finite change in a quantity... 

The ratio of the total heat output of the heat pump to the quantity of electric energy input to the heat pump is called the heat effect or coefficient of performance (COP). This value depends on the temperature gradient, but is typically around 3-4 for an output temperature of 30 °C (and can be higher). Canada specifies a minimum COP of 3.0 for all ground source heat pumps rated below 35 kW (Bouma 2002). 

It should be noted that the efficiency of die forward cycle is highest when 7j is as high as possible. Since, in practice, Tt, will always be fixed by the temperature of the surrounding atmosphere, a high efficiency corresponds to a large difference 7j — Tq. In contrast, a high coefficient of performance, or a high effectiveness of a heat pump corresponds to a small difference T, - T . 

Note that the coefficient of performance of a heat pump of a refrigerator, unlike the efficiency of a heat engine, can be greater than unity. 

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