Energy Efficiency in Chillers

For many medium to large commercial buildings the air conditioning service is largely provided by circulating cold or ‘Chilled’ water.

The principle of its application for air-conditioning is simple, being the opposite of the familiar hot water central heating found in a majority of homes in the UK.

The machine providing this chilled water is referred to as a Chiller.

Essentially a very large water refrigerator, but for many these machines are something of a mystery, as indeed is any form of refrigeration system – how does the Black Magic of using what is ultimately heat energy make things get cold?

How also is it that refrigeration systems claim an energy efficiency greater than 1? The first law of Thermodynamics states Energy can neither be created nor destroyed, only moved from one place to another. It is the second part of this Law of physics that refrigeration systems exploit, and more recently maximising that exploitation has become key to achieving decent energy efficiency from the chiller.

Vapour Compression Systems

The base principle of vapour compression systems (with compressors) is using a base refrigerant fluid that evaporates and condenses at convenient temperatures and pressures. Now remembering school day physics lessons a concept known as latent heat of vaporisation / condensation comes into play, and is largely the reason why refrigeration systems achieve an energy efficiency > 1.

Chiller efficiency

Measuring a heat energy input into a fluid within the constraints of just one state, whether gas or liquid will see a directly proportional temperature change relative to heat energy input. Such a heat input is referred to as “Sensible Heat”. But when the fluid, a liquid, reaches its vaporisation temperature the same heat energy input continues, but the temperature ceases rising whilst the transition from liquid to gas occurs. This ‘lost’ heat energy is causing the change of state, and this extra heat is known as latent heat. For most fluids the latent heat required to change state per unit volume is massive compared with that to simply cause a temperature rise, typically 10 – 20 times the energy required per oC of temperature change sensible heating requires.

Refrigeration System

A refrigeration system cheats this principle by using vapour compression to cause the sensible heat rise only within the fluid when it is compressed into a smaller volume, and hence higher pressure. So by then cooling this fluid now at higher pressure the fluid temperature will fall below the condensation temperature, so a change of state occurs, and in doing so releases the latent heat of condensation. This process happens within the “Condenser” vessel. The resultant liquid leaving the condenser is thus a tepid high pressure liquid fluid.

By then leading this to a similar vessel via a restriction the reverse effect occurs. The restriction causes the pressure to fall, this in turn causes the liquid to exceed the vaporisation temperature. The liquid has to boil, and in so doing must absorb latent heat from its surroundings, effectively the low pressure liquid becomes very cold as it boils into a gas.

By designing all the system components to work together we end up with a refrigerator, absorbing heat from the process, in the case of a chiller the water used to cool the building. Now the only energy input we have provided is the relatively smaller amount of energy required to cause the pressure and temperature rise / volume reduction at the compressor.

Over the Past 20 Years

20 years ago when energy was cheap and not considered important, Chillers were achieving an efficiency of ~ 2 so 1 kW of energy into the compressor would harvest a cooling capacity of ~ 2 kW. The past 20 years has seen development of numerous individual techniques to improve the achieved energy efficiency from the humble chiller so currently 4 is commonplace and better-designed equipment is now exceeding an efficiency of 5. Improving efficiency from 2 to 4 = 50% input (paid for !) Energy savings. 2 to 5 = 60% less energy.

What efficiency are your chillers achieving today?

©Trevor Dann - ThermOzone Ltd