We compared the performance of heat pumps driven by thermally-generated grid electricity and by a local heat engine with heat recovery. We found that, over a wide range of system parameters, the engine-driven system delivered about twice as much energy (in the form of hot water) for each unit of fuel burned as did the electrically-driven system.

Thinking about the building thermodynamically led to the proposal that we shouldn’t introduce energy to the building at a higher grade than its use requires. In practice, this is a hard aim to meet, because energy is not usually available at a wide range of grades. Unless one lives near a CHP scheme, most energy is available at a high grade, in the form of fuel or electricity. Even renewables use high-grade resources like solar radiation or the kinetic energy of air. The upshot is that we find ourselves forced to use high-grade sources for low-grade tasks. If we do so in the conventional way, for example, burning gas to heat our houses, we are abandoning some of the usefulness of the energy. How can we use high-grade energy for low-grade tasks in a way that makes full use of its potential?

The answer comes from the 2nd Law of Thermodynamics: the degradation of the energy in our fuel allows us to upgrade some energy from a source that is *colder* than our house, to a temperature suitable for heating the house. At Shettleston, an electric heat pump extracts energy from 12 C groundwater and delivers it at more than 40 C to the hot water tank. Having operated the heat pump, the energy from the electricity appears in the hot water too. The degradation of the electrical energy “pays for” the upgrading of the groundwater energy, and the result is that we use much less electricity than we would if we used ordinary heaters.

However, most of our electricity is generated in thermal power stations, which means that it has been created by a chain of degradation and upgrading of energy that sends two-thirds of the energy in the original fuel up the power station cooling tower. We can do better if we move the power station to the heat pump, by driving the heat pump directly from the shaft of the engine. The engine still produces about two units of heat for every unit of mechanical power, but now we can recover that heat and use it to heat our water.

A possible arrangement is shown in Figure 1. The heat pump is used to raise the incoming water to an intermediate temperature, and the engine cooling circuit is used to raise it to the final temperature. In this way we keep the temperature lift of the heat pump as small as we can, giving the maximum benefit from heat pumping.

 

Analysis

We did a simple steady-state analysis of a system like the one illustrated, and compared its performance to that of an electric heat pump and of a condensing gas boiler. To make comparison more straightforward, we considered the systems as parametric variants of each other. For example, we considered the gas boiler to be a heat engine with zero fuel-shaft efficiency. The parameters used are shown in Table 1.

.Table 1. Parameters of different systems for providing hot water, and the primary energy ratios for the three systems. The electricity for the heat pump is assumed to be generated thermally.

We specified performance as a primary energy ratio, that is, the ratio of the amount of primary energy (eg fuel) used to the amount of energy delivered as hot water. The primary energy ratio of the three systems is shown in Figure 2. It is clear that the engine-driven heat pump provides almost twice as much heat per unit of fuel burned as does the electric heat pump. We repeated the analysis, varying each of the parameters widely. The absolute levels of performance varied, but the relative performance of the two heat pump systems stayed more or less constant.

Figure 2. Primary energy ratios for three heating systems.

There are potential prices to be paid for the efficiency of the engine-driven heat pump. For example, the engine is complex piece of machinery with associated maintenance needs. We have to consider emissions and noise as well. The last two issues might be tackled by the use of a Stirling engine (LINK), which uses continuous external combustion of fuel.