ADVANCED HEAT PUMPS

There is, in any case, at least one application of heat pumps that may become important to those whose business it is to invest in energy sources electric utilities. Heat pumps could be used to store energy produced by power plants when demand is low, in order to tap it during peak loads.

A system now under investigation at the Rutherford Lab in Hilton, England, uses the refrigeration principle in the form it was first used to make ice as long ago as 1811.

Unlike modern refrigerators, where the refrigerant circulates as vapor in one direction and as a liquid in a separate return, the system uses only one channel between the evaporation and condensation vessels, and only water vapor travels back and forth through it. Apart from being a heat pump, it also works with chemical energy. It works like this:

There are two vessels communicating as shown above one is filled with water, the other with concentrated sulfuric acid. Theacid will absorb water vapor from the other vessel, lowering the pressure above the water and evaporating more vapor for absorption. During this phase, heat is released in the acid vessel for two reasons: First, latent heat is pumped from the water to theacid vessel, cooling the water (to 0 degrees to 10 degreesC) and warming the acid as the water condenses in it.

In addition, and more significantly, heat is released due to the chemical reaction of water mixing with sulfuric acid; the acid reaches temperatures of 50degrees to 100 degreesC. This high temperature heat can be tapped and utilized.

The process continues until the acid is so diluted that it will no longer absorb significant quantities of water vapor. At that point the energy stored in the system has been used up.

It is then "recharged" by re concentrating the sulfuric acid, i.e., by heating it so as to expel the water, which will travel as vapor back to the water vessel, where it will condense. The heat used in this distillation can be supplied by solar, wind, or wave energy, or by off peak electric power. (The heat pump from low-temperature water to high temperature acid needs no external input; it runs on the chemical energy stored in the concentrated acid.)

In practice, the system is a little more complicated: both vessels contain heat exchangers, which are covered by a surface film of water in one vessel, and of acid in the other; the two vessels hold mainly water vapor. The films on the heat exchangers are replenished from storage vessels, and the diluted acid is withdrawn to a third storage vessel for distillation in the charging process.

Sulfuric acid was chosen because it is cheap (about $50 a ton), familiar for handling in large quantities, and capable of a high storage density 0.3 kWh per litre (about 1 quart). As a heat pump, the arrangement is not particularly efficient its significance lies in the energy storage accomplished by using off peak energy to concentrate sulfuric acid by distillation, and to release the energy again by diluting it.

So far only a model with an input of 100 W has been constructed; it is being upgraded to 5 k\N', and construction of a full sized energy storage system based on this principle may begin next year. A 10 to 20 year payback time is hoped for, particularly where waste heat can be used for the reconcentration process.

More: Energy Storage Provided by Heat Pump," Electrical Review (England), 14 April 1978 .]