HEATPUMPS

HEATPUMPS:

What´s that and how do they work?

Improvements in energy efficiency, leading to lower environmental pollution, particularly by CO₂, can be achieved in many ways. The heat pump, which can be used for heating and cooling buildings as well as in many industrial processes, offers the best prospects for attaining these goals in a wide variety of appropriate applications.

By upgrading low temperature heat, enabling it to be used at a higher temperature, the heat pump can use fossil fuel resources more effectively than conventional heating methods.

The heat pump is the only known process that recirculates environmental or waste heat back into a heat production process, and therefore can dramatically reduce the demand for fossil fuels as well as NO_x, hydrocarbons, acid rain and CO₂-emissions.

Detailed studies have shown that electric- or gas-driven heat pumps would reduce emissions of CO₂ and other pollutants associated with space heating in Europe by between 30 % and 50 %, depending upon the conditions of use.

The vast majority of heat pumps operate on the vapour compression cycle , which can accommodate electric motor or gas engine drives. A growing minority of heat pumps employ what is called the absorption cycle. They predominantly use gas as fuel and are directly-fired. Many other methods exist, but are not yet widely applied.

Source: Brochure 'Heat Pumps - an option for an energy efficient and clean society' of the Commission of the European Communities.

The vapour compression cycle is used in the majority of heat pumps, and the use of electric motor drives dominates such units. The principal components in the basic vapour compression cycle circuit are shown in the figures opposite. These comprise an evaporator and condenser, the compressor and its drive and an expansion valve. Not to be omitted is the working fluid, or refrigerant, which circulates around the system.

The fluid is evaporated at low temperature and pressure in the evaporator, using heat from the heat source. The vapour is then compressed, raising its temperature before proceeding to the condenser, which gives out useful heat by condensing the working fluid at this higher temperature. The flow of the fluid through the expansion valve reduces the pressure to that of the level in the evaporator.

The net heat output at the condenser is effctively the sum of that taken in at the evaporator, and the heat equivalent of the work of compression put in by the drive. This leads to an energy 'bonus' which sets the heat pump apart from other heating methods - the heat delivered is greater than that which could result from fully effective use of the primary energy alone.

In the case of a gas engine drive heat may be recovered from the water jacket and exhaust to supplement that delivered at the condenser.

The absorpotion cycle is not expainded by me, because it would exceed our time.

How are heat pumps judged?

When you buy an automobile, you ask for the power.

Heat pumps are judges by there performance number, this number is usually bigger than 1 and means, e.g. with a number from 3, that 1 Wh electrical energy supplies the water an energy of 3 Wh. An direct heater work with an performance number from maximal 1, because they don´t use the environment heat.

Why is there a compressor in use?

The reason for using a compressor is that the refrigerant energy output becomes bigger because of the higher pressure. This circumstance is the same as it is with water. That means that the refrigerant is able to give more energy to the used medium.

Importent is that the refrigerant is converted to low pressure as it is at the evaporator, after the condenser again, otherways you will have a problem with the huge losses.

If you want to see an heat pump modell, you should go to the mechanical installations in our school.

Placement in real life:

Vocabulary:

heat pump     Wärmepumpe

environment  Umgebung

prospect        Aussicht

to attain         erreichen, erzielen

appropriate    geeignet

associate       kombinieren

the vast majority                             große Mehrheit, enorme Mehrheit

accommodate                                  hier: unterbringen, beinhalten

minority        Minderheit

expansion valve, throttl valve         Expansionsventil

not to be omitted                             nicht zu (verabsäumen, vernachläßigen) ist

performance number                       Leistungsziffer, Arbeitszahl

Quellen:

http://www.ikz.de/art_2196/9621050.htm

http://www.ikz.de/art_2296/9622082.htm

http://www.fiz-karlsruhe.de/hpn/hpn.html

http://www.ochsner-online.com/choice.htm

The affinity between the absorption cycle and the vapour compression cycle can be seen in the Figure below. An absorption heat pump contains an evaporator, condenser and expansion valve which operate in exactly the same way as those in the vapour compression system: Heat is taken in at the evaporator causing the working fluid (refrigerant) to evaporate at low pressure, and heat is released by condensation at high pressure. This type of heat pump does not necessarily need fossil fuel energy. It can use any thermal source such as waste heat and solar energy instead.

In the absorption cycle, however, there is a secondary circuit around which a liquid absorbent or solvent flows. The evaporated refrigerant vapour is absorbed into this at low pressure, liberating the heat of condensation and solution. The solvent, now diluted by refrigerant, is raised to the high pressure by a liquid pump. High pressure refrigerant vapour is then produced by the addition of heat to the mixture in the generator. The resulting heat output is, as in the case of the vapour compression cycle, greater than that added from primary energy sources.

Because the liquid solvent/refrigerant mixture is virtually incompressible, (unlike the vapour in the vapour compression cycle), the work of the pump is genuinely negligible and can be supplied from sources within the system. The primary energy source is the heat required at the generator, which is always the hottest part of the cycle. This can be provided by a gas burner. It may also use any thermal source, such as waste heat or solar energy, at a convenient temperature.

The absorption cycle heat pump may at first sight appear much more complicated than the vapour compression system. However, it is essentially a collection of heat exchangers, and this offers (a) considerable scope for cost reduction. Cost and size reductions are the goal of several national and international absorption cyle programmes.

A second feature of the absorption cycle is the frequently environmentally friendly nature of the refrigerant/solvet fluid pair. Most commonly these are ammonia/water or water/lithium bromide.

Another sorption system is the solid-gas adsorption heat pump. Although currently less well developed than systems using liquid absorbents, this offers opportunities not normally associated with more conventional heat pump systems. These include integral heat (and cold) storage and the ability to produce heat at temperature levels of up to 200-300°C. This latter feature makes the solid-gas system highly appropriate for industrial process applications.

Source: Brochure 'Heat Pumps - an option for an energy efficient and clean society' of the Commission of the European Communities.