Reflecting on a year dominated by elevator issues, safety, and modernization.

My New Year’s resolution is to stop using the term “heat pump” and instead, wherever possible, to refer to a device that takes heat from a low temperature to a high temperature as a “heat elevator.”

In my very first column for this magazine (“The Temperature Lift,” ASHRAE Journal, March 2012) I used the analogy of lifting a piano uphill to illustrate the challenge of lifting heat. You can roll the piano down to a convenient place but then you need to work hard to raise it up to another convenient place where you can then roll it down into its required location. It strikes me that you wouldn’t talk about “pumping” the piano up the hill, so “heat elevator” seems a more apt name for the temperature raising device.

The confusion created by the misuse of “pump” in this context is matched, or possibly even surpassed, by the idea that the heating efficiency of such a system can be more than 100%. For some reason many people, particularly mechanical engineers, struggle with this notion. They feel that an efficiency ought to be a measure of what was actually achieved compared to the theoretical maximum that might have been gained and so by definition must be less than 100%. Electrical engineers, however, seem to have no such problem. For them efficiency is a measure of output divided by input. In the case of heating efficiency it is perfectly feasible to deliver more heat in a heat elevator than would have been created by putting the electrical input through a resistance heater. In fact, since the work input is added to the heat that is being elevated and contributes to the heating effect delivered at high temperature it ought to be obvious to all that 100% efficiency is the minimum that can be achieved by a heat elevator, not the maximum.

To get this point across I like to use another analogy: comparing the heating efficiency of a heat elevator to the mechanical advantage afforded by a lever, pulley or gear mechanism. With these it is perfectly normal to put in a certain force and achieve a higher force (albeit acting over a shorter distance) on the other side of the device. Describing the heating coefficient of performance or the heating efficiency as a “heat advantage” gets over the mechanical engineers’ objection by speaking their language.

Of course the most beneficial application of a heat elevator is the situation where someone has a need for the heat being delivered and someone else can benefit from the cooling effect created by the removal of heat from the low temperature source. Problems arise in calculating a performance metric in this case. If the cooling coefficient of performance (COP) is 3 and the heating advantage is 400%, does that mean that there is a system COP of 7? It’s all heat transfer, but adding the two effects together like this is like adding apples and bananas—it’s all fruit but not the same. Another way to think of it is that the cooling effect is like someone digging a hole in the ground. They are very interested in the length, width and depth of the hole, but they don’t really care about the pile of sand that they are creating in the process. On the other hand someone building a fabulous sand sculpture doesn’t really care where the sand came from but they are totally focussed on the size, shape and form of the pile that is being created. Of course it makes no sense to add the hole in the ground and the sand sculpture together, just as adding the cooling effect and the heating effect together is meaningless.