High Performance Heat Pumps
In previous articles, I presented a technical overview of the refrigeration cycle. However, even with this background, it can be difficult to recognize the vast utility of this technology, especially in the context of domestic heating. For the purpose of effecting cooling, either in order to keep food from spoiling or to moderate the temperature of living spaces in hot climates, the refrigeration cycle is the obvious and often only practical choice. Early work in the field was heavily motivated by the former and was quickly adopted for the latter objective.
However, the refrigeration cycle has also been extensively deployed for the purposes of heating, taking the form of a wide variety of heat pump systems. The benefits of this technology are less tangible in this context, as it may seem that the utilization of such a complex process for this application is a waste of engineering potential or resources. In order to understand why this is not the case, we need to first appreciate a few facts about conventional heat sources commonly used for household heating. These overwhelmingly involve some form of combustion, referring to a high-temperature, exothermic, or energy-releasing chemical reaction, involving a fuel and an oxidant. In most space heating applications, that oxidant is simply atmospheric oxygen. Perhaps the oldest and most recognizable fuel used is wood, while most modern furnaces are fed by a familiar engineered petrochemical, such as fuel oil or propane.
The greatest quantity of energy that can be extracted from the burning of a given fuel is well-defined by its composition. That is, one cannot simply increase the heat output of a fuel-burning appliance without limit; there is only so much chemical energy stored in the particular fuel. The design of oil and gas burners has improved continuously over the decades, but every advancement yields diminishing returns, thereby reducing the potential for future development. Even electrical resistance heating is bound by the same rigid efficiency limit, though due to the underlying thermodynamics, greater efficiency is possible when compared with combustion-based alternatives. However, it is often uneconomical to heat with electricity in this manner due to the relatively high cost per unit energy. In contrast, the use of the refrigeration cycle for heating is not so severely constrained.
The capabilities of a heat pump system are measured by the coefficient of performance (COP), rather than the typical efficiency percentage given for most other energy-consuming appliances. The COP is defined as the ratio between the amount of thermal energy released into the heated space and the amount of electricity used by the appliance. Modern air-source heat pumps, under favorable environmental conditions, can offer a COP as high as 5. This means that for every unit of energy supplied to the device, up to 5 times that amount of energy is available to heat your home. For comparison, the effective COP of an electrical resistance heater is 1.
It is important to realize that, in the most scientifically rigorous sense, the COP is not truly an efficiency metric. From the first law of thermodynamics, we know that efficiency is capped at 100%, since energy cannot be created or destroyed. One cannot ever get out more than one puts in. However, through the magic of the refrigeration cycle, heat pumps appear to skirt this limitation. This perceived conflict is easily explained by the fact that a heat pump transfers or ‘pumps’ energy between thermal reservoirs, rather than transforming one form of energy into another, as is the case for combustion or electrical resistance heating. In so doing, heat pumps seemingly cheat the laws of thermodynamics and achieve sufficient heating while minimizing the consumption of resources.
The Solar Initiative continues to offer subsidies supporting the adoption of heat pump technologies for residential applications on Block Island. To learn more about our programs, please click here or email Wade Ortel at email@example.com