Hybrid Energy Systems in Future Low Carbon Buildings
 
Scope  
Background  
Micro wind  
Heat pumps  
Heat recovery  
Solar thermal panels  
Pv  
Bipvt  
Chp  
EarthToAir heat exchange  
Passive design  
Thermal storage  
Design concept  
Hybrid concept  
Methodology  
Modelling tools  
Case study results  
Environmental impact  
  



Heat Pumps Background Information
Contents
Introduction

The general principle of heat pumps has been known for a long time, refrigerators and air conditioning units were the first common apparatus using the same principle of the heat pump. Heat pumps were developed commercially several decades ago but due to cheaper heating system options using fossil fuels (gas, oil) they have had limited development.

Heat pumps are considered as a renewable energy source; with the current concerns about climate change and fossil fuel depletion, heat pumps have started since the 1990s to become popular again.

How it works [1,2,3]

A heat pump takes heat from a ‘warm’ place and drops it in a ‘cold’ place. As per the second law of thermodynamics, such a process is only possible with a work input into the system, in the present case this work is in the form of electricity consumption. The heat pump uses a fluid refrigerant (having specific properties) to transfer heat from hot to cold.

A heat pump is made of 4 main elements:

  • Compressor
  • Condenser
  • Expansion valve
  • Evaporator
  • Compression phase:
      - The refrigerant is in gas phase, its pressure and temperature go up
      - The compression requires most energy (electricity) input in the entire cycle, the rest comes from the pump.
  • Condensation phase:
      - The gas at high temperature and pressure goes to the warm area (typically air or water) and delivers a heat load
      - As a result the temperature and pressure drop and the gas condenses into liquid
  • Expansion phase:
      - The liquid is still at high pressure and expands to a lower pressure, its temperature then goes down at the same time
  • Evaporation phase:
      - The low temperature and pressure liquid then goes to the ‘cold area’ (which is nevertheless warm relatively to the temperature of the refrigerant) and receives a heat load.
      - The liquid starts evaporating into gas as a result.
In the heating mode the warm area will be the inside of the house at room temperature while the cold area will be the ground or external air or water. In cooling mode this is reversed.

The heat pump therefore to transfer an amount of heat while consuming a lower amount of electrical heat, usually the output energy is 3 to 5 times the input. The ratio between the heat output and electrical input is called the coefficient of performance (COP).

Types of heat pumps

The COP depends on the temperature difference between the two areas (cold and the warm). For instance when heating a water tank with a ground heat pump, the COP will depend upon the difference between the ground temperature and the water temperature.

The COP is provided by the manufacturer and measured for a given temperature difference to where the heat is extracted from. If temperature is colder then the actual COP will be lower. In other words the COP the heat pump is running at is not constant and depends on the actual conditions of usage.

Air source heat pump (ASHP or AHP)

- The heat is extracted from the ambient air during the evaporation step and released inside the dwelling. The medium where heat is delivered will be usually either air or water.

- Air source heat pumps are common in mild climates. They are not recommended in cold climate as they can be quite inefficient if ambient temperature comes near freezing point and will usually stop working a few degrees below. Condensation is also an issue when outside ambient temperature falls too low.

- Unless the ambient air temperature remains mild the COP will therefore drop below levels that make a HP beneficial. One of the main advantages however is that they are cheaper and easy to install.

- AHP can be located inside or outside the building.

Ground source heat pumps (GSHP or GHP)

- The heat is extracted from the ground and released inside the dwelling. As for the AHP the medium where heat is delivered will be usually either air or water.

- The key benefit of this system is that during the cold season the ground temperature remains much higher than the ambient air temperature, so the COP is better than for AHP. GHP are relatively simple and therefore well adapted to rough climates. In fact they were first used extensively in Sweden.

- In order for the refrigerant to circulate in the ground there are 2 possible schemes:

  • Horizontal design
      - This is relatively inexpensive. The depth is typically 0.6 to 1.2 m
      - Heat extracted in the ground will re-generate itself as long as a minimum distance is kept between other nearby installations. At low depth, heat regeneration comes mostly from solar energy and rain water
  • Vertical design
      - One or more wells are dug up to 80m deep
      - This scheme is more expensive but less space is required. Drilling is the major cost item.
      - Some regulations apply usually
Water source heat pumps

- In this case the heat is extracted from a water source, such as a river, underground water or a lake. Water has excellent thermodynamic properties when it comes to heat transfer or storage.

- This scheme is nevertheless less common because water is seldom available nearby where the installation is required.

- Regulations apply as well.

Specifications

Typical COP of AHP will be about 3. However if temperature goes below freezing the COP will be lower, and may go below 2.5
Ground source heat pumps have generally a COP of 4 to 5. In Japan the best heat pumps now reach a COP equal to 6.

Electricity generation through conventional processes is typically a 30% efficient process, assuming low transmission losses. This means 1 kWh of electricity is ‘environmentally’ worth at least 3 kWh of primary heat.
A heat pump should therefore be operating at a COP preferably higher than 3 to balance the carbon footprint of the electricity it requires. Obviously if the heat pump is powered from renewable sources of electricity then things are different.
In France financial help can be obtained to install a heat pump only if the COP of the installation is higher than 3.3.

Heat pumps are more effective when heating well insulated dwellings, so that the peak demand is reduced. Their efficiency will also depend on the type of internal installation to deliver the heat. Low temperature systems such as under-floor heating will allow to improve the efficiency.

Typical power output range from a few kW to > 50kW

Typical heat pump lifetime will be at least 20 years
Underground system lifetime should be over 40years

Benefits of heat pumps

  • Free renewable heat:
      - When the heat pump consumes 1kW of electricity it delivers 4 kW of heat (for a COP=4) so that’s 3kW of heat coming from the ground which are transferred to the dwelling.
      - In other words 100% of the heating load of the dwelling can be met consuming 4 times less energy in electricity.
  • Potential carbon savings compared with other heating methods
  • Reduction of the dependence on fossil fuels
  • Heat pumps can be used easily within a hybrid scheme:
      - They can be coupled within other heat sources
      - They can heat up both space and domestic hot water
  • Heat pumps do not require fuel storage
  • Heat pumps also present no significant safety issues
  • Many heat pumps are reversible, meaning they can provide a cooling load in the summer if required.
Issues associated with heat pumps

  • Heat pumps still consume electricity. Unless they operate at a high COP or are powered by renewable electricity they will still represent a relatively high primary energy consumption.
      - To obtain a good COP will impose limitations on how high is the temperature where heat is delivered. Typically heat pumps will perform better if they deliver heat at temperature around 45C
  • The quality of the installation is therefore critical.
  • Heat pumps are not necessary suited for all buildings. There will be space requirements for GHP.
  • AHP installed outside will generally create noise
  • AHP will usually require an alternative source of heat in case outside temperature is too low
  • The use of GHP implies that heat must regenerate itself during summer, so the captor area must be large enough and installations cannot be put too close together. Otherwise, it is necessary to use it in reverse to re-inject heat in summer (by cooling the building for instance)
Costs

Typical costs provided per m2 of dwelling area to be heated
  • GSHP:
      - Horizontal     ~ £70-100 /m2
      - Vertical     ~ £ 140-200 /m2
  • AHP
      - Most types     ~ £ 60-90 /m2
  • Operation cost
      - ~ £3- £5 /m2 /year
      - The operation cost is highly dependant on the quality of the building insulation.
Installation costs for standard dwellings will therefore range from £7000 to £13000 for GHP
The cost may be reduced in case the HP is installed at the same time other construction is taking place.
The final cost will also depend on the internal heat delivery system as well as the control system, while under-floor heating preferred the cost may prove prohibitive in certain cases.



References:

· Afpac.org - aides financières
· Aprofeel.com - pompe à chaleur
· Afpac.org - qualité formation
· Xpair.com - pompes à chaleur et géothermie
· EnergySavingTrust.org.uk - Ground source heat pumps
· Heatpumps.org.uk
· Heatpumps.co.uk - Ground source heat pumps
· Certita.org - liste pompes
· GeoHeat.oit.edu - Geothermal heat pumps
· European heat pump network