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Heat pumps, moving energy and converting temperatures.
by Hakan Falk, "Energy Saving Now", April. 2004
It can be argued that energy use is always a matter of moving the energy from one media to an other, this is if you take a strict interpretations of the laws of energy. Fore practical purposes, we are making distinctions between use, generation, storage, conversion and moving. This will keep the professors in language and physics eternally busy in criticising poor practical engineers like me, because it is no way that we can get the terminology right all the time and it is as many possible translations as professors.
Refrigerators or heat pumps exists for all kind of temperatures and applications, they are all energy movers. We are only dealing with applications in buildings and dwellings. The working principles are not explained here in details, so if you need more information, or brush up your knowledge, we recommend to visit Air Conditioners at "How stuff works". The heat pump is a descriptive name of a reversed AC cooling unit. What we want to explain is the design envelope for heat pumps, because this is not that often described.
Principles.
All materia can take the form of solid, liquid or gas, each at a defined temperature and pressure and going from one state to an other will require a large amount of energy. We call this transformation points the freezing and boiling points. Normally this is used for separate or mixing of materia. Around a quarter of the world population is practising chemical engineering on a daily basis. The most common forms of moving energy in HVAC, is by physically moving and mixing materia with different temperatures, but the same form. To make this movement more efficient, the freezing boiling points are used to store/release energy and most frequent used is the boiling point.
By manipulate the pressure, the boiling point will be at a higher or lower temperature. A very common application of this, is the pressure cooker, that is rising the pressure and the boiling temperature. The result of the higher water temperature is a significant faster cooking of the food in the cooker, water that escapes will do that as steam, after the pressure valve.
Absorption method.
This was the original heating manipulation method for the "refrigeration revolution", a Swedish invention. It gasify the medium on the inside by outside heating elements elements and pressure valves and can be driven by a multitude of sources. The most common media is ammoniac. The problem with this process is that it needs the same heating energy that it removes from the cooling space. Therefore it has been replaced by the compression method in its general applications. It is however still popular in applications that requires the system to be driven by multiple sources, like Fridge for RV and boats.
A very interesting and new application for the absorption method, is the use for solar driven AC. An Austrian company have develop a media and method, with a lower boiling point that is suitable for thermal solar panels. Since thermal solar panels is 3-4 times more efficient than current PV cells and significantly cheaper, it is a very interesting application. The capacity also grows with the need.
Compression method.
Compression method manipulate the boiling point by mechanically changing the pressure with a compressor. This method is less energy demanding than absorption method and will move 3.1 times more energy than its use, under ideal circumstances. This method have taken over the major part of cooling and heat pump applications.
A compressor have traditionally freon as media and show a bell like efficiency curve. For heat pumps the maximum efficiency is with an outside temperature between 8 to 10 degree Celsius and then falling to a bottom where the used energy is the same as moved energy.
The two major type of compressors are the rotation and the piston compressor.
Rotation compressor.
The rotation compressor is predominant in the stand alone AC/Heat pump units, because its cheaper production cost. It have the maximum of 2.9 times at 8 to 10 degree Celsius and will often have its minimum a 0 degree Celsius. Higher quality rotation compressors can have its minimum as low as -5 degree Celsius.
The average life time of a rotation compressor is around 8 years.
Piston compressor.
The piston compressor is predominant in professional AC and Heat pump applications and is also used in industrial equipment, It have the maximum of 3.1 times at 8 to 10 degree Celsius and will often have its minimum a -15 degree Celsius. A typical value at 0 degree Celsius is the moving of 2 times the used energy.
The average life time of a piston compressor is 20 to 30 years.
Heat pump considerations.
The working ranges for the different type of compressors.
The source and delivery. The typical stand alone AC units is air to air and that is efficient in climates where outside temperatures seldom goes down to 0 degree Celsius. In colder climates, a system earth to water is by far the best efficiency. That means that the collection circuit is water pipes in earth and under freezing level. This guarantee constant temperatures between 5 to 12 degrees, depending on geographic location. The maximum working conditions can be achieved.
On the delivery side, radiation is also much more efficient delivery than heating air, but the highest delivery temperature are 45 degree Celsius or slightly lower. The low delivery temperatures make it ideal for heated floors. If used in radiators, they have to be larger sizes than what is practice, because of the low delivery temperatures.
Preferable are DC compressors who permits capacity regulation with variable speed. This give clear efficiency advantages, compared with an AC compressor regulated with start/stop, 20 to 30% less energy use with variable speed is not uncommon. It also make the parameters for the room climate easier to manage.
Energy saving and cost considerations.
It is considerable savings of energy and costs, when using heat pumps. To give you a picture, we will use cost savings as units and descriptions. Our example is taken from the situation in most European countries.
The following comparisons, use blower heat fans as starting pont and considers each change on its on,
- Change to oil filled electric panel radiators from blowers, 10 to 15% savings.
- Central pulsating thermostat, with clock and ventilation protection, around 10% to 15% savings.
- Change to water filled radiators with central heating and central thermostat, 25 to 35% savings.
- Using central high efficiency heating with oil instead of electricity give 40 to 50% savings.
- Using heat pump with air collector in warmer climates and earth collector in colder, 50 to 65% savings.
- Producing hot water with oil instead of electricity, around 40% savings.
- Production of hot water with heat pump recover from ventilated air. 65% savings.
- Production of hot water with solar panels (80%) and heat pump, around 90% savings.
Above savings is not accumulated effects, but the difference between one installation to another can be up to 80% in costs and energy consumption. If you then add usage of low tariffs, if available, the savings can be up to 90%. This comparisons are between worst and best case. Worst case being a badly insulated house with resistor heating of air in ventilation system and water production, best case a well insulated house with heated floors, heat pump and solar/heat pump recovery for hot water production.
The worst case situation is very common and very difficult to fit in with a total use of renewable energy sources. For the best case, it is very feasible to use solar (thermal and PV), wind and biofuel systems for the necessary energy production.
Hakan Falk
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