Geothermal, Heat Pumps & Solar Thermal Flashcards
What is a ground source heat pump (GSHP)?
Heat pumps draw heat from the ground via a ground collector or a ground probe.
~ 2.5m down, the earth remains at a constant temperature about 10 to 15°C in the UK. We can make use of this fact to transfer this latent heat to your home using a ground source heat pump.
What is an air source heat pump (ASHP)?
They extract heat from the air outside which varies in temperature with the weather.
They then transfer the heat from the air into your home using a heat pump.
Heat pumps draw heat from the air using a collector (heat exchanger) usually with a large fan.
Describe the principle of ‘extra’ energy in a heat pump?
A heat pump requires electrical energy input to work but the thermal energy provided exceeds that inputted - this is referred to as the ‘extra’ energy.
It works through a reverse refrigeration cycle.
Are air source or ground source heat pumps better?
Air source heat pumps are cheaper but the heating is less reliable since the outside temperature varies a lot.
GSHPs are more expensive to install than ASHP due to the need for the digging of wells or trenches in which to place the pipes that carry the heat exchange fluid. However, they operate at a higher efficiency.
Describe the cycle in which a heat pump works to supply heat to a house.
- Warm heat transfer fluid passes through high surface area system giving out heat to its surroundings (usually inside the building e.g., radiators or underfloor heating).
2.The expansion valve controls the pressure of the fluid. The pressure is high in the hot region in the system, but drops in the cool region.
3.The fluid is allowed to expand in the cool region and becomes a gas in this high surface area system and in doing so adsorbs heat from its surroundings (usually in ground outside).
- The compressor uses electrical power to compress the gas converting the fluid back into a warm high pressure liquid ready to be recycled.
What is the typical COP range for a ground source heat pump?
3 and 4.5
When are GSHPs most suitable?
For homes or non-residential buildings, where typically a field of vertical bore holes is drilled.
They are most efficient when the temperature difference between the heat source and the heat demand is small. This allows them to operate at a higher efficiency.
Best suited to under floor heating systems (38°C) rather than radiator systems which typically run at 70°C or hotter.
What is the typical COP range for an air source heat pump?
3 and 4.5 but 1 to 2 on colder days
What are the main 5 advantages of heat pumps?
- Low Operating Cost. They provide domestic hot water & heating at very low cost.
- Low Maintenance Costs. They have few mechanical components therefore they are reliable, easy to service and not likely to fail.
- Durability. Heat pumps should last much longer than conventional Heating, Ventilating, and Air Conditioning systems (HVAC) because they are protected from harsh outdoor weather. The ground heat exchanger and its associated piping has an expected life of over 50 years
- Low Noise. These systems are among the quietest ever designed.
- Reduced Emissions. They do not require external venting and they do not pollute the air.
What are the main disadvantages for heat pumps?
- The main disadvantage of using a GSHP is the considerable disruption during its installation. Less of a problem for ASHP.
- For GSHP the ground area needed may not be available. The length of the ground loop depends on the amount of heat needed. Normally the loop is laid flat, or coiled in trenches a few metres deep, but if there is not enough space available a vertical loop to a depth of up to 100 metres can be used. Access for digging / boring machinery is required.
- In new buildings it is usually most cost effective to install the system during the development.
- ASHPs have variable performance depending upon the air temperature – least efficient in winter.
What is geothermal power?
The power extracted from heat stored in the earth.
What is geothermal energy & where does it originate from?
It originates from the original formation of the planet and from radioactive decay of minerals.
What are the 2 uses for geothermal energy?
- Geothermal heating (direct)
- Geothermal electricity (indirect)
True or false: Direct heating is less efficient than electricity generation and places less demand on the heat resource.
False - direct heating is more efficient
Why can’t all geothermal energy be used for direct heating rather than for electricity generation?
Co-location of heat
source and demand
are not always
possible.
How is electricity generated from geothermal energy? What are the 3 main types of plant?
Wells are drilled into a geothermal reservoir. The wells bring the geothermal water to the surface, where its heat energy is converted into electricity at a geothermal power plant.
The main 3 types are:
1. Dry steam
2. Flash steam
3. Binary cycle
Describe a ‘dry steam’ type geothermal power station.
The first geothermal power plants were dry steam systems.
Steam from the geothermal reservoir is routed directly through turbine/generator units to produce electricity. An example of a dry steam generation operation is at the Geysers Regionin northern California.
Requires a suitably hot dry steam reservoir.
Describe a ‘flash steam’ type geothermal power station.
Flash steam plants are the most common type of geothermal power plants in operation today.
They use water at temperatures greater than 360°F (182°C) that is forced under high pressure into the generation equipment at the surface.
The hot high pressure geothermal fluid comes up to the surface and enters flash drum where the pressure is suddenly reduced, allowing some of the hot water to convert or “flash” into steam.
This steam is then used to power the turbine/generator units to produce electricity.
The remaining brine that is not flashed into steam, and the water condensed from the steam, is generally pumped back into the reservoir (maintains the fluid level in the geothermal zone)
Requires a hot reservoir.
Describe a ‘binary cycle’ type geothermal power station.
Binary cycle geothermal power generation plants differ from dry steam and flash steam systemsbecause the water or steam from the geothermal reservoir never comes in contact with the turbine and generator units.
the water from the geothermal reservoir is used to heat another “working fluid,” which is vaporized and used to turn the turbine/generator units.
The geothermal brine and the “working fluid” are each confined in separate circulating systems or “closed loops” and never come in contact with each other.
What are the advantages of a binary cycle power plant?
By using working fluids that have an even lower boiling point than water they can operate with lower temperature waters (225°Fto 360°F).
They also produce no air emissions.
What are the advantages of geothermal power?
- Low carbon dioxide emissions compared to conventional energy sources.
- Heat energy is free & not intermittent.
- As technology improve thenumber of exploitable geothermal resources will increase.
- Not as much pollution as fossil fuels.
- Easy to predict the power output with a high degree of accuracy.
- Renewable energy source.
- Increased research and exploration is finding new suitable sources of GT power.
What are the disadvantages with geothermal power?
- It is location specific
- Gases can be released into the atmosphere during digging (or continuously from older plant).
- Geothermal energy runs therisk of triggering earthquakes due to changes in pressures in subterranean fluid.
- High capital expense.
- Geothermal fluid needs to be pumped back into the underground reservoirs faster than it is depleted to maintain sustainability.
What is meant by solar thermal energy?
Using the solar energy incident on the planet to directly generate useful thermal energy (i.e. converting sunlight to heat - not electricity).
How does sunlight generate heat?
Heat is the vibration of molecules or atoms.
Light consists of photons, which are annihilated upon being absorbed. The energy excites electrons within the absorbing material and usually this results in the electrons then relaxing by giving up their energy by creating phonons (quanta of vibration or heat)
What is passive solar? Give an example.
The use of incident solar energy without the use of mechanical or electrical devices.
It usually involves simple ideas incorporated into building or appliance design in order to utilise the maximum amount of natural heat and/or light energy. It does not compromise the intended function of the building or appliance.
For example, the location of ancient villages/modern homes. Building villages on south facing cliffs to maximize heat and light gain in winter. Positioning under a rock overhang minimizes gains in the summer and protection from rain and snow.
What are the 3 types of passive design?
- Direct Gain
- Indirect Gain
- Isolated Gain
What is direct gain passive design? What passive and active controls can be used?
Where sunlight enters the building. For example, commonly occupied spaces and openings of a building are orientated in a southerly direction (in N hemisphere) to maximize any direct benefits.
- Passive - overhanging skirts on the roof provide shade in high summer but let sunlight in via windows during winter.
- Active - shutters on windows keep out sunlight on very sunny days and therefore prevent overheating. Windows and doors can increase/decrease circulation control the distribution of heat. For example, dark surfaces absorb heat and shiny surfaces reflect light and thermal mass allows heat to be retained.
What is indirect gain passive design? Give an example.
It captures heat but light remains external. If direct gain is likely to cause large fluctuations in heat and light and make the building uncomfortable it may be better to use indirect gain.
With indirect gain, sunlight is stored as heat in the large thermal mass. This provides more consistent heat day and night, reducing fluctuations in temperature.
For example, a Trombe wall:
- Transparent or translucent outer wall
- Thin air gap (up to 80ºC)
- Inner wall with dark absorbing surface on a high specific heat capacity material to store heat. Isolated from the ground and any metal framework to reduce heat loss.
- Optional one way ventilation slots allow control of the heat distribution
What is isolated gain passive design? Give an example.
Where heat (and light) captured are transferred. This is often retro-fitted as an add-on structures but may be embedded in the initial design.
For example, courtyards, conservatories & atria.
Describe how an atria works as an isolated gain passive design? What should be considered?
Atria behave as internal courtyards. They usually used in commercial buildings and have a glazed roof or front.
The choice of materials & finishings are very important.
Overheating can be controlled by ventilation and adjustable shading.
They also provide protection from the weather and act as a buffer at the entrance to the building between the outside and inside the main building.
Natural heat and light distribution throughout the building is aided by reflective surfaces and ventilation stacks.
Describe how a courtyard works as an isolated gain passive design? What should be considered?
Courtyards are an isolated open space within a building, found in hotter climates (e.g. Mediterranean)
Many different designs e.g. fountains or pergolas are used to provide cool air.
In summer, evaporatively cooled air falls in the courtyard and is drawn through the house to replace external rising hot air. Active air and water circulation systems can also be used to keep the courtyard cool.
In winter the deciduous plants covering the pergola no longer shade the courtyard allowing heat and light in.
The now exposed surface can absorb heat and reflect light. Reduced airflow retains heat.
Choice of materials & finishings are very important.
Describe how a conservatory works as an isolated gain passive design? What should be considered?
These are a glazed enclosure attached to a house. They can provide heat and light when the sun is bright, but can also be isolated using internal doors to prevent thermal loss when not.
They act as a buffer between the outside and the main building.
Best oriented facing south in the northern hemisphere.
Double glazing will help to insulate the conservatory whilst still allowing natural light and heat in.
In modern designs, what are the 5 main passive design features?
- Apertures – windows : let sunlight in.
- Absorbent materials – dark surfaces : convert light to heat energy.
- Heat store – bricks, concrete : retain the heat and smooth out temperature fluctuations.
- Distribution system - ventilation, reflective surfaces : distribute the heat and light.
- Control mechanisms – sun shades, opening windows, shutters : enable passive or active control of environmental conditions.
What are the properties of glass?
It is transparent to visible light and opaque to the IR radiation re-radiated from surfaced heated by visible light (Greenhouse Effect).
What is the daylight factor?
The daylight factor (DF) is used to express how much of the externally available sunlight enters a building.
What is the daylight factor dependent on?
Three main components:
1. Direct sunlight,
2. Externally reflected light
3. Internally reflected light
It is also dependent on:
1. The number and size of openings/windows.
2. South facing is best but anywhere in the range from 30° towards east or west. Larger openings provide more larger heat gains and losses.
3. External over-shading (trees/buildings) can reduce light available.
4. Sun path diagrams prior to construction help avoid such issues.
5. Trees and nearby buildings can provide heat retention benefits by sheltering from wind.
