Microgeneration Flashcards
Microgeneration definition
The small-scale generation of heat and power
Micro-electricity
Microelectricity technologies refers specifically to small devices that are capable of producing electricity. The only two examples that should be stated in the examination are: Solar PV and Wind
Micro-heat
Microheat technologies refer specifically to small devices that are capable of producing heat. The only three examples that should be stated in the examination are: Solar thermal, Ground source heat pumps and Biomass
Reduced energy cost impacts
- Less electricity used from the National Grid.
- Less heating oil used to heat a development.
- Less natural gas used if a gas pipeline is available locally.
- Less fuels purchased for heating, such as turf, logs and coal.
Because the implementation of the microgeneration technology reduces the use of these there are reduced energy costs to the home or community.
Enhanced security of supply
When the occupants of a house or a local community install a microgeneration system they are in a position where some or all of the following apply:
- They require less electricity from the National Grid.
- They require less natural gas.
- They require less heating oil.
- They require less miscellaneous fuels such as turf, logs and coal.
Heat pump basics
A heat pump is a system that aims to transfer energy (specifically thermal energy) against a natural temperature gradient. Energy transfers naturally from a high temperature region to a low temperature region.
Radiator roles
Absorbing Energy on the Low Temperature Side: The Evaporator
Radiating Energy on the High Temperature Side: The Condenser
Component 1; The Evaporator
The evaporator takes in a liquid, called the refrigerant, at colder temperature than the air outside. It absorbs thermal energy from the air and transfers into a gas. This gaseous refrigerant is still at too low a temperature for use.
Component 2; The Compressor
The refrigerant, in gaseous form, is pumped to the compressor. Here the gas is compressed, increasing the pressure and critically the temperature of the gas. The gas, now at sufficient temperature, is pumped to the condenser.
Component 3; The Condenser
The condenser now transfers this thermal energy to the water being circulated through the heat exchanger. The water from the hot water tank is circulated from the tank to the heat exchanger and then back to the top of the tank at high temperature. The refrigerant in the condenser loses thermal energy and starts to condense back into a liquid. It is now a mixture of gas and liquid at high temperature. This refrigerant is now pumped to the expansion valve.
Component 4; The expansion valve
The expansion valve allows the gaseous part of the refrigerant to expand, reducing its temperature and condensing fully into a liquid but now at lower temperature than the evaporator end of the system. It is now pumped to the evaporator for the process to continue.
Air source heat pump advantages
Can be easily installed in existing buildings as well as new builds.
Lower costs than ground source heat pumps.
Does not require a large area of ground around a building such as a large garden.
Air source heat pump disadvantages
The air temperature varies much more than the ground temperature during the year. Air temperature can vary from 30 °C in summer to -15 °C in winter, a range of 45 °C. Ground temperatures below 0.5 m in depth vary around 6 °C in comparison.
The lower temperature air contains less thermal energy and this large range limits the COP (see LO7) of air source heat pumps which are lower than that of a ground source heat pump. The heat pump makes a humming noise similar to an air conditioning unit.
Ground source heat pump
A ground source heat pump extracts thermal energy from a refrigerant that circulates in a loop in the ground. At a depth of more than 0.5 m, in NI, the ground has a relatively consistent temperature throughout the year between 6 °C and 12 °C. This relatively small range of consistently higher temperatures, when compared to the range of air source heat pumps, enables ground source heat pumps to exhibit larger COP values (see LO7). While the system is more effective, offering reduced repayment periods, they do cost much more and are likely to range between £10,000 to £18,000 (as of 2019). The additional costs are primarily based on the requirement for excavation (digging) of the ground on the development, to allow the pipework to be installed and buried below ground level.
Ground source heat pump advantages
The ground at 0.5 m depth or below maintains a more consistent higher range of temperatures, compared to an air source heat pump, which results in a higher COP.
Repayment periods are smaller.
Ground source heat pump disadvantages
Larger upfront costs.
Large area of ground is required to install the pipework loops in the ground.
Energy flow through a heat pump
The law of conservation of energy states that, ‘energy cannot be created or destroyed merely transformed from one form to another’
As a result of this, the following applies to all systems in terms of energy: Energy into the system = Energy out of the system
Maximising the effectiveness of a heat pump
Two key points need to be considered when aiming to maximise the effectiveness of the system:
- QC is energy that is free.
- W is electrical energy that must be supplied to the heat pump system and must be paid for.
As a result of this, every heat pump system needs to ensure that the following criteria are met:
- The value of QC must be maximised.
- The value of W must be minimised.
Money matters
The implications of COP on the costs of heating a house or development are staggering.
For a COP = 3: For every unit of electricity purchased three units of thermal energy are delivered by the heat pump. A customer only pays for 1/3 of the energy delivered and so bills are reduced by 2/3.
For a COP = 4: For every unit of electricity purchased four units of thermal energy are delivered by the heat pump. A customer only pays for 1/4 of the energy delivered and so bills are reduced by 3/4.
Power and energy
Calculations for a heat pump could use the terms energy or power. Before undertaking calculations in exercise 6.7 it is critical that a solid understanding of power is achieved to get these calculations correct.
Power = Energy transferred / Time taken P = E / t
Where:
P = Power in W.
E = Energy in J.
t = time in s.
Power is simply the energy used per unit time and is measured in watts, W. 60 W merely represents 60 joules per second. It is evident from Figure 6.18 that the energy into the heat pump per second is equal to the energy leaving the heat pump per second.
As a result of this it can be stated: Power into the heat pump = Power out of the heat pump
