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Does “heat pump” double electrical energy?

Dugna? Ottikku retti? Ponzi scheme?

Claims of one unit (KWHr) of electricity producing 2 units of cooling and 3 units of heat – getting the useful energy of 4-5 times – how does this work? Is it defying laws of physics?

The law of conservation of energy: Energy cannot be created or destroyed, it can only be changed from one form to another” – can we defy that?

Let’s see.

One of the most common apparatus in the world is an air-conditioner. Let’s consider what happens in that. Inside the air-conditioned room is colder than outside. That means, the heat from inside of the room needs to be pushed out – is it not? But, heat flows from hotter place to colder place! How does this reverse flow happen?

This is like a water pump – a pump pushes water from a lower level to a higher level – reverse of what usually can happen – usual flow is from a higher level to lower level. That is accomplished by “work” done by the pump. This is precisely the reason why a heat pump is called a heat “pump” – it pumps heat from a colder place to a hotter place. We provide electrical energy to the equipment in the air-conditioner – but, what happens is the heat energy from inside the room is pushed (pumped} to the outside. Now, if you go near the outside unit of an air-conditioner, you would have realized that it is hotter than the atmosphere. It needs to be so for pushing the heat outside. In a heat pump system – the heat given out to the atmosphere in air-conditioning system is used to heat water. That is about it.

So, one unit of electricity is used to “pump” 2 units of heat from a colder place to a hotter place! What is gotten is not what is given – but like water – what is obtained is what is pumped. One unit if electricity is not “converted” to 2 units of heat – but it “pumps” 2 units of heat from a colder place to a hotter place through work delivered by a heat “pump” which is operated by electricity. Pay for heating and get cooling free!

What is all this “Buy One – Get One Free” kind of talks in industrial and commercial heating / cooling side? Is this a marketing gimmick? Is it true? Again, going back to the earlier post on “pumping” of heat from a colder place to a hotter place, let’s define what happens.

In a hot place – say Chennai – average outside day temperature of – say 35 deg C. what we need inside the room is – say 25-degree C. Heat needs to be “pumped” from 25 deg C to 35 deg C. For this to happen the Air-conditioner needs to deliver “cold” at a much lower temperature than 25 degrees C – for air in the room to get cool. Routinely – air-conditioners work in the 6 to 8 degrees range. Let’s look at outside – if the heat needs to be pushed outside – the air conditioner needs to have a temperature over 35 degrees – air-conditioners typically deliver 45-50 degrees. A “lift” in a typical home air-conditioner is 6 degrees (cooling side) to 50 degrees (heating side). In an air-conditioner scenario also, it is possible to “harness” the heat given away by the external unit. But, at 50 degrees C, it is difficult to use that heat.

Now, come to heat pumps: The heat pumps operate at a “lift” of 20 degrees (cooling side) to 60 degrees (heating side). Some of the modern heat pumps can deliver 20 degrees {cooling side) to 90 degrees (heating side). It is the hot side that is used for heating water – to say 55 degrees to 60 degrees (or up to 85 degrees In modern heat pumps). How about the cold side? 20 degrees can be put to use? At least in a factory environment? Or, in a hotel room? Or in any process that requires cold temperatures of 20 degrees C? Of course – yes.

Voila – we have answered how this seemingly physics-law-defying Ponzi scheme is not a Ponzi

scheme. It is not the conversion of electricity into heat like in conventional electric heaters, but “pumping” of heat.

Here is the limitation and its possible solutions:

One side must be primary: In heat pump system – the primary objective is to heat the water to 60 deg c (or 85 deg c in case of modern ones) – so, if the cooling side does not operate, the heating side also will stop. Hence, if our primary objective is heating, we need to ensure that the heat can be “pumped” even if the cooling side does not operate. This is a common failure by many designers who have hybridized the system without backing up for situations when heating and cooling are not operating simultaneously.

What have we done?

What we have done in such cases is to take the colder side and back it to a sump or a heat sink that can take the heat and get cool. In one case, we have used this to cool the tank that provides input water to a cooling tower – this way the cooling tower also gets more efficient, saving energy.

In a hotel room kind of a scenario – while this “free” cooling can reduce the load on the air conditioner,  we need to have back up air-conditioners.

Applications of cooling: Factory shop – to provide a better environment for workers – in this case, it can be optional, and hence they get the benefit only when the heat pump is running.

We can create an “oasis room” that has a few seats and a water fountain which area is maintained cold by the heat pump’s cooling side.