Reversible Heat Pumps

Air conditioners are sometimes called heat pumps because they pump heat from the inside of a room or house to the outside. This cools the inside and makes the outside a little bit warmer, though not warm enough to make much of a difference. (But as discussed in the section on the urban heat island effect, a huge number of air conditioners operating at the same time, as in a large city, might have an impact on the city’s temperature.)

In the winter, people do not need to cool the house, but rather keep it warm. The traditional manner of doing this is by using a heater. But over the last few decades, engineers and scientists have started looking at another option. What if an air conditioner could

be made to work in the reverse direction? Then it would pump heat from the outside to the inside. The result is that the temperature inside would rise.

66 Time and Thermodynamics

Similar to air-conditioning, this kind of heat pump would require energy, because during the winter it would be moving heat from the cold exterior to a warmer interior, in opposition to heat’s natural flow. But the idea is exactly the same as air-conditioning, and the same procedure would work. All that is needed is to reverse the circulation of the working fluid.

Many homes and businesses are choosing reversible heat pumps to maintain year-round comfortable temperatures. (Some people simply call these devices heat pumps.) They are not the same as air conditioners because they must be reversible. There is not much advantage of a reversible heat pump over a traditional (one-way) air conditioner, because they work the same way; the advantage is that heat pumps prevent the need for installing both an air conditioner and a conventional furnace or heater. Furthermore, heat pumps are usually more efficient heaters. Although extracting heat from the cold air outside and moving it to the inside of the house requires energy, it does not generally require as much as the burn- ing of oil and gas or sending a lot of electrical current through a heating element.

The problem with reversible heat pumps is that in the winter they sometimes need to be run in air-conditioner mode for brief periods of time. This is particularly necessary when the temperature goes below freezing, because ice can form on the coils outside. To keep from blowing cold air into the house, the machine uses heating elements to warm the air during these short periods when it is running in the wrong direction. Although these periods of time do not last very long, they cut down on the overall efficiency of the heat pump.

An excellent way of getting the most out of heat pumps would

be to have access to a gigantic object that is at a relatively constant temperature throughout the year, neither very cold in winter nor very warm in summer. A heat pump could efficiently draw heat from the object during winter and deposit heat during the summer. Due to its size, the object would not change temperature much in the process, so it would provide a nearly inexhaustible source of heat in the winter and a heat “sink” or “drain” during the summer. This source surprisingly exists. It is called Earth.

Heat and Technology 67

The Earth’s interior is hot, as volcano eruptions repeatedly demonstrate. The center of Earth is about 4,000 miles (6,400 km) below the surface, and geologists believe that its temperature is about 7,000°F (3,870°C). (Though no one knows for sure, because no one has been able to measure the temperature at Earth’s center directly.) Heat flows upward toward the cooler crust. The soil not far below the surface has a nearly constant temperature—it is in thermal equilibrium, balanced by heat from below and the cold, exposed surface layer of dirt and rocks. The temperature of this subsurface region depends on latitude (north-south location), ranging from 50–70°F (10–21.1°C).

Heat pumps that take advantage of this source are called geo- thermal heat pumps (geo refers to Earth). They should become more common in the future. Thermodynamics places strict limita- tions on the manner by which people can heat and cool homes, so everyone needs to take advantage of any opportunity to increase efficiency.

Reversible Heat Pumps