Chapter 6 Flashcards
What is the definition of Microgeneration?
The small-scale generation of heat and power
What is the definition of microelectricity technology?
Small devices that are capable of producing electricity.
What is microheat technology?
Small devices that are capable of producing heat
List 2 microelectricity technologies
1- solar pv
2- wind
List 3 microheat technologies
1- solar thermal
2- ground source heat pump
3- biomass
Identify 4 main benefits of home/ community microgenerations.
1- Reduced energy costs
2- contributions to environmental targets
3- enhanced security of supply
4- Financial incentive
Explain the role of the evaporator in a heat pump
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.
Explain the role of a compressor in a heat pump
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.
Explain the role of an expansion valve in a heat pump
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.
Explain the role of the condenser in a heat pump
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.
Name 2 advantages of an air source heat pump
1- Can be easily installed in existing buildings as well as new building.
2- Does not require a large area of ground around the building
2 disadvantage of air source heat pumps
1- Air temperatures varies a lot more than ground temperatures and the low temperature in the air does not carry as much thermal energy.
2- The pump makes a humming noise.
What liquid circles in a loop in a ground source heat pump and how deep is this loop.
Refrigerant circulates to extract thermal energy from the ground and the loop is usually 0.5M deep.
What degree of temperature does the ground change throughout the year?
6° and 12°
2 advantages of a ground source heat pump.
1- 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.
2- Repayment periods are smaller
2 disadvantages of a ground source heat pump
1- Larger up front cost compared to air source heat pump.
2- Large area of ground is required to install the pipework loops in the ground.
What is stage 1 in a heat pump and what does this do?
Stage 1 is the evaporator, the refrigerant (liquid) enters the evaporator at a lower temperature than it’s surroundings outside, as such energy transfers from the surrounds to the evaporator the energy inside the refrigerant increases, and changes state into a gas
What is stage 2 of a heat pump and what does it do?
Stage 2 is the compressor, The refrigerant passes from the evaporator to the compressor in a gaseous state. But the gas exists at low temperature. The compressor compresses the gas and in doing so increases the internal energy of the gas further as it does work on the gas. This increases the pressure of the fluid but crucially increases the temperature of the fluid to a level above the higher temperature side of the heat pump system which is the interior of the house. This refrigerant now can be passed to the condenser.
Stage 3 of a heat pump and what does it do?
Stage 3 is the condenser, The refrigerant now passes to the condenser. The refrigerant, now a gas, is at a much higher temperature than the external environment and so thermal energy transfers from the refrigerant to the external surroundings. As thermal energy transfers away from the refrigerant it loses energy and its temperature will start to decrease. It will start to condense and leave the condenser as a mixture of gas and liquid.
What is Stage 4 of the heat pump and what does it do.
The expansion valve, expands the refrigerant so the refrigerant does work. As the refrigerant does work it has less energy afterwards and so it starts to condense turning all of it into a liquid.
What is stage 5 of a heat pump
Repeat the process
What is a heat pump?
A heat pump is a system that aims to transfer energy (specifically thermal energy) against a natural temperature gradient.
How does energy transfer naturally.
Energy transfers naturally from a high temperature region to a low temperature region.
What is a heat pump designed to do?
A heat pump is designed to transfer energy from a low temperature region to a high temperature region.
