Heat Pumps: A Look Backwards and Forward

To talk about heat pumps it is essential to start from the beginning, i.e. to discuss heat itself. Today the notion of heat is so commonplace that the term is used in a number of senses without much distinction being made among them. Heat is what fries or boils an egg; heat is what you feel on your skin when the sun shines on it; heat is what makes a struck match burst into flame; heat is what turns a barroom conversation into a brawl or what you feel under your collar when someone disparages your car, your clothes or your pet.

Heat, as it will be discussed here is a manifestation of disorganised molecular energy. The more intense the disorganized energy, the higher the temperature rises. The bigger the object endowed with this temperature, the larger the quantity of thermal energy becomes. Speaking in a strictly technical sense, heat is thermal energy in transit from one object to another: Heat is what flows from a hot object to a cooler object.

The History of Heat

In the late 1700s, the development of the steam engine was well underway. The most active development was in Britain where it was first employed to pump water out of mines. This new technology began to displace the earlier sources of pumping energy… mostly water power and literally horsepower… which had previously restricted energy-intensive activities to very limited geographical areas.

This development was pioneered for the most part by imaginative technicians: toolmakers, instrument makers, tinkerers and talented improvisers. The first devices to use the power of steam used its negative pressure, i.e. the partial vacuum produced when steam in a closed vessel is cooled and condensed.   Anyone who has ever cooled down a pressure cooker will have experienced this phenomenon as the strange combination of sucking sounds that occur just before the lid can be removed. The progression to the use of steam at positive pressures quickly produced technical advances. It also gave steam power its growing reputation as a dangerous and unreliable way to produce power as a result of burns, metal fractures and explosions.

It was in this milieu of frenetic technology development that the young French scientist/engineer Nicolas Leonard Sadi Carnot began his efforts to understand from fundamentals exactly how fire produced power. One of the many difficulties he faced was that there was no real understanding of heat at the time.

Heat was thought to be a manifestation of a weightless fluid called Caloric. What is really remarkable is that Carnot identified the basic influences which limited the efficiency of engines and then reasoned that the most efficient heat engine must be one that is reversible.

Everyone has experienced the magic of steam power, most often in the mechanical poetry of the steam railway locomotive… hissing, snorting, menacing… the iron horse was a well-chosen name indeed.

The Carnot reversed engine became the foundation stone of the refrigeration industry and in later years the air conditioning industry. The “other half” of the heating-cooling machine, the heat pump, was to remain a technology largely without an application for many decades. There were exceptions to this generalization of course. The ability of a mechanically driven machine to produce useful heat was recognized very early. One of the candidates put forward for designation as the inventor of the heat pump is Peter Ritter von Rittinger, a prolific Austrian inventor who worked in the mining industry. His use of the heat pump was to supply the heat needed to extract salt from brines. This event is dated to 1856, very early in the development of mechanical heating and cooling.

Early Heat Pump Technology

Early technology enthusiasts in many places used heat pump technology to heat houses as early as the 1920’s. However, heat was so much easier and cheaper to produce from coal, oil, wood or early types of town gas that “pumped heat” remained largely a laboratory curiosity for many years. The change in this outlook began with the disaffection for the postwar consumer-centred culture expressed in the hippy movement of the late 1950’s, the “Free Speech Movement” of the early 1960’s and other elaborations. This era saw establishment researchers being lectured by counterculture gurus on the merits of indigenous and eco-architecture…building with adobe blocks, with bent poles and animal skins, with waste materials such as glass bottles… and many other departures from standard practice. Praise was also heaped upon other exotic departures from the commonplace: solar panels, wind turbines, recycling and using heat pumps for warming buildings.

The widespread worry about oil scarcity was enough of a stimulus to the AC unit makers to add “Heat Pump” to their unit designations, whether there was any real engineering behind the claims or not.

Heat Pumps Today

Today the know-how of the heat pump has evolved to a point where there are products available for all dwelling sizes from small flats to sprawling mansions in leafy demesnes. They come in versions using a bewildering variety of heat sources: outside (ambient) air, exhaust air, groundwater, stream flow, lake and even seawater, earth and even some using solar heat. The range of sizes runs from a few hundreds of thermal watts up to more than 10 megawatts… a range extending from the domestic refrigerator up to behemoths providing heat to entire towns.

The application range has spread very far and wide. Heating and cooling and humidity control of buildings is probably the biggest single sector. Drying is another big field. Anything from pharmaceuticals to food, in quantities from grams to tonnes, the separation of water from a product is another logical place to deploy this marvellously efficient technology. Think of commercial laundries, of grain in storage silos of thousands of tonnes, of industrial kilns for wood drying, paint drying… the list is endless.

The Future of Heat Pumps

Many of these early refrigerants were corrosive, noxious, narcotic, poisonous, flammable, or had several of these undesirable attributes. The answer to these difficulties appeared to come from technology in the 1930’s in the form of the freons: chemically stable, odourless and nonflammable gases.

The other frontier is that of temperature, or more precisely the heating-side reservoir temperature. Today it is possible to buy heat pumps capable of supplying heat up to about 110°C. Existing technology is capable of extending this limit up to about 130°C, but no single market niche requiring this capability has been identified to date. The other dampening factor is that the theoretical efficiency of the heat pump process decreases as the hot-side temperature rises.

To read more on this topic, check out this article on environmentally-friendly CO₂ heat pumps.