Refrigeration and Criogenics Heat pumps

Institute of Power Engineering and Fluid Mechanics Division of Refrigeration and Air Conditioning Systems

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What is a heat pump?

A heat pump is a machine or device that moves heat from one location to another via work. Most often heat pump technology is applied to moving heat from a low temperature heat source to a higher temperature heat sink.

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Historical overview The first person ever to came with the idea of heat pumping was William

Thomson (Lord Kelvin). In 1852 he described an open air system with piston compressor, which he named as „heat multiplicator”.

The air is the working fluid. It is sucked from the environment to the expansion cylinder where it is expanded and cooled. Then it flow through the heat exchanger where is absorbs heat, up to the temperature of environment. After that, it is compressed again up to the atmospheric pressure. Then, since its temperature is now above environent it it used to heat up the room.

Rys. 1 – outside air 2 – inlet cylinder, 3 – heat exchanger, 4 – transmission, 5 – steam engine, 6 – outlet cylinder 7 – heated room

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Historical overview

Because of high demand and at the same time availability and high efficiency, refrigerators and cooling systems (especially vapor compression cooling systems) quickly become very popular among end users (domestic appliances).

On the other hand, for many years, heat pums raised very limited interest, when compared to more traditional methods of house heating (which relied on low fuel prices. The popularity of heat pumps typically increase because of:

  Energy and fuel crises;   Fuel prices increase;   Development of newer and safer technologies, refrigerants and

materials ensuring safe and problemless work;   Decrease of equipment used in refrigeration technolgies;   Increased popularity of ecological movements encouraging limited

energy use and development of environment free technologies.

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Development of heat pumps

The real development of the heat pump started in 1930s. In 1927 Thomas Graeme Nelson Haldane build the first heat pump capable of heating up

the house. His device was an vapor compression ammonia system. In 1938 in Zurich, Switzerland 175kW heat pump was installed in order to heat up office

builiding. This device was using nearby river as a low temperature source, providing temperature of the medium about 70oC.

In 1940s a large heat pump unit was installed in Norwich, England. It was also usin river water and reached the COP about 3. In that decade, 2.4MW heat pump was installed in London, for year round air conditioning and heating of Royal Festival Holl.

In following years, in the USA, England and Switzerland several heat pumps are installed using different heat sources, including waste heat of industrial processes. This is when the name „heat pump” was used for the first time: G. Flugel in Germany used the name: „die Warmepumpe” and independently F. Kraus in the USA introduced „heat pump”.

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Development of heat pumps

Further development of heat pumps was related to increased popularity of synthetic refrigerants (1950s). Lower prices, smaller size and higher reliability translated into increased demand. Heat pumps become popular heating equipment for domestic use.

Thanks to convenient weather conditions, heat pumps become very popular in the USA. In 1970s, about 850k heat pumps were installed in this country. In 1980s almost 30% of all suburbian houses had installed some kind of heat pump.

The demand is also visible in Europe, especially in Germany, France, Italy, Sweden and Norway. Many refrigeration system developers started mass production of the heat pumps. Since 1990s, heat pumps become more and more popular also in Poland.

Nowadays, heat pumps can be observed in many places, in both domestic and industrial aplications.

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The efficiency The heat pump is used to warm a closed space (i.e.

house) with the rejected heat, while the refrigerator is used to maintain a low temperature in a closed space and the rejection of heat is just a part of the operation – not its purpose.

For refrigerator QL is the desired output, while for the

heat pump QH is the desired output.

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The efficiency The efficiency of a heat pump fall significantly at low

temperatures. Therefore, most of air-source heat pumps require a suplementary heating system such as electric resistance heaters or an oil or gas furnace. Since the operating temperatures of water-source or ground-source based systems are more stable such additions are not required.

The heat pump is most competetive in areas that

have a large cooling load during cooling season and small heating load during heating season.

The heat pump is least competetive in areas where

the heating load during heating season is very large and the cooling load during cooling season is small.

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Different types of heat pumps

  Vapor compression heat pump systems   one refrigerant:

  electrical,   gas fired,   exhaust engine,   high temperature (multi stage);

  with a mixture   zeotropic,   non-azeotropic;

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  Sorption heat pumps:

  sorption;,   resorption;

  complex vapor compression and absorption systems;

  thermocompressor heat pumps;   ejector heat pumps;   others:

  Thermoelectic, rotary, etc.

Different types of heat pumps

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Bivalent heat pump

  A heat pump to be described as economic needs COPHP at least 3   Bivalent (more than one heat source) heat pumps can be connected in

series (left) or in parallel (right)

T

parownik

CHR

woda głębinowa

woda gruntowa

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II

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woda gruntowawoda głębinowa

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Heat pump with sources connected in series Heat pump with sources connected in parallel

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Heat sources

  natural   artificial

  Upper – energy is absorbed, the exergy is low   Lower – determine application of the heat pump, high exergy

In perferct case – isothermal heat source, losses are neglected.

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Lower heat source

Lower heat source – defines the pump type and determines its application. It should have clearly defined qualitative and quantitative properties:

Qualitative Quantitative

availability

corrosion potential

temperature and its changes in time

exergy and its changes in time

investment and operational cost

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Heat source

Typically small heat pumps are installed in houses with sources:   Air   Ground   Solar energy

Sometimes, but not often:   Underground water reservoirs   Wells

If available:   Rivers   Lakes   Seas

Abosrption of energy cannot influence other users of the source.

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Heat source

Heat pumps are generally much more expensive than other heating systems, but in the long range they can save considerable amount of money, because they could significantly lower heating bills.

The most common energy sources of heat pumps are:   Atmospheric air   Water   Soil (geothermal)   Waste heat

The COP of heat pumps is between 1.5 to 4 depending on the particular system and its energy source.

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Heat source

The energy cannot be transfered from high temprature source due to the losses during transport.

Lower heat source has to be present at the location of installation. This is important for large installations as well. Some special heat sources – require anti-corrosion materials   Salty underground water reservoirs   Sea water   Vapor in a drying room   Presence of SO2 i CO2 + condensation of humidity Sometimes it is a significant economic problem.

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Heat source

  The higher the temperature – the more efficient and reliable heat pump   The changes during the year, month and evern during the day, has to be

taken under consideration when evaluating heat pump capabilities.   It is necessary to determine sources coherence, especially:

  Possible significant temperature changes.   If the availability meets the demand.

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Coherent and incoherent heat sources

  Coherent heat source – gives more heat when this heat is really needed.   Incoherent heat source – gives more heat when the demand for heat is lower, eg.

The Sun is incoherent… in winter when demand is higher, the density of energy provided by the Sun is lower.

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Heat sources

Artificial – the heat is a result of some technological process (the temperature 290-350 K, sometimes higher, not depended on the year period)

The most of natural heat sources is incoherent (air, water, earth)

Cost – investment, operation, maintenance:

For 20 kW:   Air 1250 EUR   Water from the well 2500 EUR   Ground 500 EUR   Solar energy 750 EUR   Typical cost of deep drilling 5000 EUR / 1 m It is very hard to determine costs related to the artificial heat sources.

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Perfect heat source

Properties of the perfect heat source:   Infinite heat capacity (renewable source)   Possibly high and constant temperature   Without contaminants and corrosion inductors   Easily and cheaply acessed   available at the location of installation   coherent

It doesn’t exist !

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Heat sources

Secondary heat pumps Primary heat pumps

Waste heat of industrial processes Renewable energy sources

  Air   Exhaust gases   Exhaust fumes: from combustion of gases, from combustion of solid fuels. •  Condensate   Vapor   Water

  Atmospheric air   Groud   Water: underground surface deep underground reservoirs urban   Sun radiation   Geotermal energy   Wind energy

Air heat pump

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Heat exchanger for ambient air

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Atmospheric air

Heating of the air at the inlet with solar energy.

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Surface water reservoirs

Surface water reservoirs can be used as a lower heat source of a large heat pump systems. Typically, we use energy resources of larger rivers and lakes.

The energy stored in surface waters comes from the exchange between water and the air or ground.

Even rivers with small flow can transport significant amounts of energy. We can use e.g. 1/5 of the flow with corresponding temperature drop of 5K>

The energy can be absorbed at many locations along the river.

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Surface water reservoirs

Temperature distribution over the year: a) rivers 1 - Wisła (Kraków Bielany), 2 - Wisła (Warszawa Nadwilanówka), b) lakes: 1 - Mamry (Przystań), 2 - Myczkowice (Myczkowo)

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Ground In the surface layer of the ground (about 10m deep), the heat of solar radiation

and the heat exchanged with the atmosphere is accumulated. Deeper, more than 20m, both the energy from the Sun (radiation) and from the Earth is stored. At 10m depth, the temperature of the ground is constant, and typically 1K higher than the temperature of the air.

The type of the ground determines its usefulness (sand, clay, gravel) and humidity (the higher, the better heat transfer ability). Typically, to abosrb the energy stored in the ground, ground heat exchanger are used (collectors). The shape and position of the collector, significantly influence heat transfer.

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Ground

An example of a ground heat exchanger for the heat pump.The pipes are installed 1-1,5 m below the surface.

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Grunt

Temperature difference between ground and year average.

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Grunt

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Grunt

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Grunt

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Grunt

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Grunt

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Grunt

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Ground heat exchangers

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Ground based heat pump installation

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Heat pump - winter

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Heat pump – summer