Geothermal, Heat Pumps & Solar Thermal Flashcards
(92 cards)
1
Q
What is a ground source heat pump (GSHP)?
A
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.
2
Q
What is an air source heat pump (ASHP)?
A
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.
3
Q
Describe the principle of ‘extra’ energy in a heat pump?
A
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.
4
Q
Are air source or ground source heat pumps better?
A
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.
5
Q
Describe the cycle in which a heat pump works to supply heat to a house.
A
- 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.
6
Q
What is the typical COP range for a ground source heat pump?
A
3 and 4.5
7
Q
When are GSHPs most suitable?
A
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.
8
Q
What is the typical COP range for an air source heat pump?
A
3 and 4.5 but 1 to 2 on colder days
9
Q
What are the main 5 advantages of heat pumps?
A
- 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.
10
Q
What are the main disadvantages for heat pumps?
A
- 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.
11
Q
What is geothermal power?
A
The power extracted from heat stored in the earth.
12
Q
What is geothermal energy & where does it originate from?
A
It originates from the original formation of the planet and from radioactive decay of minerals.
13
Q
What are the 2 uses for geothermal energy?
A
- Geothermal heating (direct)
- Geothermal electricity (indirect)
14
Q
True or false: Direct heating is less efficient than electricity generation and places less demand on the heat resource.
A
False - direct heating is more efficient
15
Q
Why can’t all geothermal energy be used for direct heating rather than for electricity generation?
A
Co-location of heat
source and demand
are not always
possible.
16
Q
How is electricity generated from geothermal energy? What are the 3 main types of plant?
A
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
17
Q
Describe a ‘dry steam’ type geothermal power station.
A
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.
18
Q
Describe a ‘flash steam’ type geothermal power station.
A
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.
19
Q
Describe a ‘binary cycle’ type geothermal power station.
A
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.
20
Q
What are the advantages of a binary cycle power plant?
A
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.
21
Q
What are the advantages of geothermal power?
A
- 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.
22
Q
What are the disadvantages with geothermal power?
A
- 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.
23
Q
What is meant by solar thermal energy?
A
Using the solar energy incident on the planet to directly generate useful thermal energy (i.e. converting sunlight to heat - not electricity).
24
Q
How does sunlight generate heat?
A
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)
25
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.
26
What are the 3 types of passive design?
1. Direct Gain
2. Indirect Gain
3. Isolated Gain
27
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.
28
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
29
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.
30
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.
31
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.
32
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.
33
In modern designs, what are the 5 main passive design features?
1. Apertures – windows : let sunlight in.
2. Absorbent materials – dark surfaces : convert light to heat energy.
3. Heat store – bricks, concrete : retain the heat and smooth out temperature fluctuations.
4. Distribution system - ventilation, reflective surfaces : distribute the heat and light.
5. Control mechanisms – sun shades, opening windows, shutters : enable passive or active control of environmental conditions.
34
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).
35
What is the daylight factor?
The daylight factor (DF) is used to express how much of the externally available sunlight enters a building.
36
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.
37
Why is distributing & retaining heat important in passive design?
Heat distribution system is important to spread the heat evenly throughout the entire building. This can be achieved either passively or actively by circulation fans.
The distribution of heat can occur by radiation, conduction or convection.
Insulation is very important when considering passive solar design in order to prevent excessive heat loss. Insulation can be used in lofts, cavity walls, double glazing etc.
38
Why is absorbing & storing heat important in passive design?
Absorbent surfaces gain heat energy when sunlight falls on them e.g. the roof, floor or walls of a building. It is best to use materials which have good absorbing properties. Dark objects absorb well, whereas light coloured objects scatter most of the light incident on them off in random directions.
The energy (Q) is stored by placing a material with a high specific heat capacity (c) in thermal contact with the absorbing surface. e.g. masonry and concrete.
Containers filled with water (very high c) may be used in walls, ceilings and floors and to increase the thermal capacitance in comparison to air.
The thermal mass stores the energy absorbed in daytime and releases is slowly keeping the building warm overnight.
Care should be taken to avoid overheating in summer and heat loss in winter.
39
Name 2 passive solar devices
1. Solar desalination - makes fresh water from salt water.
2. Solar drying - passive solar devices used for the drying of foodstuffs & wet clothes.
40
What are the 7 advantages of passive design systems?
1. Good designs result in reduced electricity/heating bills
2. Simple in their design
3. Little or no maintenance required
4. Usually inexpensive if incorporated into initial building design
5. Pay-back periods are short to moderate.
6. Reduces emissions in comparison to conventional heating/lighting.
7. Reduces dependence on fossil fuels by a large amount
41
What are 4 disadvantages of passive design systems?
1. Can be expensive if retrofitted to an existing dwelling.
2. Reduced efficiency if dwelling is not south facing.
3. Nearby obstructions such as trees and buildings may reduce the passive heat and light gains.
4. Dependant upon the local climate.
42
What are the 3 main components of a solar water heater?
1. The solar collection panel – placed on the roof of the building oriented facing the sun to maximise the energy absorbed.
2. The circulating/pumping system - keeps the transfer fluid moving through the system, transporting the energy to where it is required. Water is commonly used as the transfer fluid in the simplest systems but other fluids like methanol and propanol are used where freezing may be a problem.
3. The storage tank – this is usually an insulated tank that stores the heated water until it is needed.
43
What is the most common type of solar water heater?
Evacuated tube
44
Describe a unglazed solar water heater.
No top covering therefore high heat loss. Only used for low heat applications e.g. swimming pool heaters.
45
Describe a flat plate solar water heater.
Heat loss is greatly reduce by the insulation provided by the glazing. The glazing also protects the system from the weather.
More than one layer of covering may be added to further reduce heat loss.
They can be used to heat the domestic water supply or for space heating in homes.
Temp <80 °C.
46
Describe a flat plate solar air heater.
They are similar to the flat plate solar water heaters but heat air rather than water. Therefore they are similar in design to a Trombe wall.
They are less efficient than flat plate water heaters.
The warm air is used to heat the building or water supply.
47
Describe an evacuated tube solar water heater
The transfer fluid (usually not water) flows down copper pipes inside a pair of concentric glass tubes. There is a sealed vacuum between the two glass tubes.
The surface of the inner glass tube is coated with a dark absorbing material.
48
What features of the evacuated tube solar water heater make it so popular?
1. The evacuated tube provides very good thermal insulation due the vacuum whilst allowing the solar radiation in.
2. The whole system is enclosed in a glazed frame to provide further insulation.
3. The transfer fluid can reach temperatures of >180 ºC and then goes to a heat exchanger where it is cooled and is recycled
4. These systems can be used in cooler climates as because the transfer fluid is not necessarily water and therefore freezing need not be a problem.
5. These devices can be used in conjunction with a micro-turbine and generator to produce electricity with an overall efficiency of 12%.
49
What are the 2 types of pumping system for solar water heaters?
1. Electrical - use a sensor on the solar collector to determine when the pipes are hot. When they are, the fluid is pumped to a heat exchanger where the transfer fluid cools down by heating up water for the domestic water supply which is stored in an insulated tank. An electric immersion coil can be added to the tank to maintain the water temperature when insufficient solar energy is available .
2. Thermosyphon - storage tank is placed outside the building at the top of the solar collection panel. Water flows from the tank down into the pipes at the bottom of the collection panel where it is heated by the sun. The warmer water rises by convection exiting from the top of the collection panel back into the storage tank. Top up immersion heaters can also be used on these systems.
50
What are the 7 advantages of solar water heating?
1. Applicable to most buildings
2. Thermosyphon systems are low/moderate cost
3. Life span relatively long term for both pumped and thermosyphon systems.
4. Grants and subsidies available often available
5. It is possible to make your own system.
6. Zero emissions for the thermosyphon system and very low emissions for the pumped systems
7. Negates the need to burn a large amount of fossil fuels therefore reducing carbon emissions, saving resources and money.
51
What are the 6 disadvantages of solar water heating?
1. Erosion/fouling may take place.
2. Installed systems may be intrusive.
3. Pumped systems are moderate/high cost
4. Pumped systems may require more maintenance.
5. Low to moderate efficiency.
6. Potential hazard to the environment or people if a toxic transfer fluid is used.
52
What are large scale solar systems used for?
To produce power in more diverse forms – usually to be converted into electricity.
53
How is heat transformed into electricity?
Heat is converted into mechanical motion using some type of heat engine and then subsequent conversion of the mechanical motion into electricity using a generator.
54
How do heat engines work? How do we get maximum efficiency?
By taking in heat at a high temperature and ejecting it at a lower temperature. Therefore they are all subject the limits of the 2nd law of thermodynamics.
Low temperature systems usually have lower efficiencies. This is the ideal (or Carnot) efficiency, in reality the actual efficiency will be less that this due to losses in the system.
55
How is a Rankine cycle different to a Carnot cycle?
It includes a turbine
56
What are the 4 types of large scale solar concentrator?
1. Parabolic Linear Reflector
2. Fresnel Linear Reflector
3. Point Focus Tower
4. Point Focus Dish
57
Describe a Parabolic Linear Reflector.
A long linear line parabolic reflector focus sunlight onto a pipe where the transfer fluid reaches temperatures < 500°C. They are mainly used for power generation.
The moving mirrors have to be highly efficient in order to reflect and focus the sunlight onto the pipe. Dust and dirt are therefore a problem. Storms can also result in broken mirrors.
Uneven heating of the collector pipe can cause problems due to thermal stresses as it expands and contracts at different rates on each side.
58
How does a Parabolic Linear Reflector work?
A heat exchanger transfers the heat from the transfer fluid (synthetic oil) to water to make superheated steam.
The superheated steam passes through a Rankine cycle steam turbine and thus drives a generator.
Direct steam generation was considered, but problems arose due to the low thermal conductivity of the stream generated in the collector pipes, resulting in pipe damage.
Mean day efficiency values of 18% have been achieved.
59
Describe a Fresnel linear reflector.
Flat plate reflectors focus sunlight onto a pipe where the transfer fluid reaches temperatures < 500°C.
Rows of flat reflection mirrors positioned below a stationary cavity receiver containing a single absorbing pipe.
The reflection mirrors track the sun in elevation and therefore concentrate the sunlight onto the collector. A secondary glass reflector refocuses any light which misses the collector pipe therefore heating the transfer fluid.
The operating temperature is ~450ºC.
60
Describe a Fresnel compact solar concentrator.
Mirrors reflect radiation onto multiple towers where flat cavities contain multiple pipes filled with transfer fluid.
The mirrors are slightly curved in order to produce a fine focus.
The problems associated with the parabolic trough due to uneven heating of the pipe do not affect the Fresnel reflectors (second reflector).
61
Does a linear or compact Fresnel solar concentrator generate more power?
Compact
62
How does a Fresnel Linear Reflector work?
1. Sunlight hits the linear reflectors.
2. It is reflected onto the collector where it heats the water.
3. The steam produced is at high pressure & drives a turbine.
4. The steam is condensed and recycled.
5. The turbine is connected to a generator which produces electricity.
6. The electricity is converted to the correct voltage and supplied onto the national grid.
63
Do parabolic or Fresnal linear reflectors have better efficiency? Why?
Parabolic.
Fresnel collectors have only about 70% of the thermal performance of a parabolic trough per aperture area due to the gaps between mirrors.
64
Give an advantage and disadvantage of a parabolic reflector rather than Fresnal.
Advantage - It is experimentally and commercially validated.
Disadvantage - Costs are greater (investment, maintenance, electricity).
65
Give an advantage and disadvantage of a Fresnal rather than parabolic reflector.
Advantage - It is cheaper (investment, maintenance, electricity).
Disadvantage - It is not experimentally and commercially validated.
66
Describe a point focus dish solar concentrator.
It is a parabolic solar concentrator that can track the sun in both elevation and azimuth. It focuses sunlight onto a point target that reaches temperatures of up to 500°C.
The receiver contains a transfer fluid such as H2 or He which when heated can drive a gas turbine or is can consist of a Stirling engine.
They are very expensive therefore are yet to take off commercially.
67
True or false: A Fresnel linear reflector has a higher efficiency than a point focus dish.
False - Point focus is higher.
68
How does a Stirling engine work?
As the gas heats and cools, its pressure rises and falls. The change in pressure drives the piston inside the engine, producing mechanical power, which in turn drives a generator and makes electricity.
1. Power piston has compressed the gas, the displacer piston has moved so that most of the gas is adjacent to the hot heat exchanger.
2. The heated gas increases in pressure and pushes the power piston to the farthest limit of the power stroke.
3. The displacer piston now moves, shunting the gas to the cold end of the cylinder.
4. The cooled gas is now compressed by the flywheel momentum. This takes less energy, since when it is cooled its pressure dropped.
69
Describe a point focus tower solar collector
Flat plate reflectors focus sunlight onto a point target that reaches temperatures of 800°C.
The heliostats track the sun’s movement and reflect it onto the tower
Each heliostat has a curved reflective surface area
The central receiver on the tower is designed to minimise radiation and convection losses, and houses the absorber.
The absorber contains a transfer fluid. High pressure & hot transfer fluid is used to drive a turbine and generator to make electricity.
70
What are the 6 advantages of solar concentrators?
1. Moderate/high efficiency
2. Large scale and high output
3. Displaces a large amount of fossil fuels utilised by the public and therefore, saves resources and money.
4. Reduces emissions in comparison to conventional systems.
5. Fuel used to heat the transfer fluid is free.
6. Storage can be incorporated in the system.
71
What are the 9 disadvantages of solar concentrators?
1. Intermittent source
2. Large areas of land required
3. High construction costs.
4. High maintenance costs.
5. Expert technical staff required.
6. Majority of systems are still in prototype stages.
7. Erosion/fouling of mirrors may take place.
8. Potential hazard to environment or locals if toxic transfer fluid is used.
9. Large structures may be considered and eyesore.
72
What are the 3 main components of a solar chimney?
1. Large transparent solar collector sloping towards centre.
2. Tall hollow chimney
3. Wind turbines inside the chimney.
73
How does a solar chimney work?
1. Collectors absorb heat from sun.
2. The heated air rises, and due to the slight slope of the collector it moves towards the central chimney.
3. The hot air rising up the chimney generates suction which draws cooler air into the collector to subsequently be heated.
4. The warm air rising up the chimney drives the wind turbines to produce electricity.
74
What are the collectors in a solar chimney set up made out of?
Glass or plastic.
Water sealed black bags or pipes may be place on the ground inside the solar collector in order to add thermal mass and enables operation both day and night and also to improve the overall capacity of the plant.
75
What are the 9 advantages of a solar chimney?
1. Direct and diffuse radiation collected.
2. Will operate in cloudy regions.
3. Day and night operation possible
4. Low maintenance
5. Simple building materials
6. No transfer fluids used
7. Moderate/high efficiency
8. Can be scaled to give high output
9. Displaces a large amount of fossil fuels and therefore saves resources and reduces emissions.
76
What are the disadvantages of a solar chimney?
1. Large areas of land required.
2. Moderate construction costs.
3. Not resistant to earthquakes.
4. Expert technical staff required.
5. Erosion/fouling of the collector may take place.
6. Majority of systems are still in prototype stages.
7. Large structure may be considered and eyesore.
77
What are the 2 types of solar pond?
1. Freshwater
2. Salt gradient
78
Describe how a freshwater solar pond works.
Usually used for smaller domestic scale set-ups. ~ 10m deep.
The pond is filled from a storage tank during hours of sunlight. The water is trapped between a black absorbing layer at the bottom of the pond and various transparent insulating layers on top.
Operated in batch or continuous mode: In the continuous mode, the shallow solar pond can be compared to a flat-plate collector in which the water is flowing continuously through the system and distributing the thermal energy evenly.
In the batch mode, the water is pumped into the pond during daylight hours and at night is passed into a storage chamber.
79
What other alternative designs for freshwater solar ponds can be used?
The water storage tank as an integrated part of the pond therefore making the pond much deeper. This results in fewer pipes and ancillary equipment required which can reduce cost dramatically.
Increased insulation covering the sides of the pond as well as the surface to reduce heat loss. Bottom insulation can also be applied but is done so to a lesser extent.
During daylight hours the top insulation layer can be reduced in order to encourage maximum exposure to radiation.
A foam insulation spray which can be applied at night and removed from the surface during daylight hours may also be used to maximise efficiency.
80
Describe how a salt gradient solar pond works.
They are usually used in larger scale set-ups for district heating & power generation.
Black plastic on lining the pond absorbs sunlight.
The storage layer is warmed up and can rise to approximately 90°C causing a salinity difference and hence density difference between the surface layer and the storage layer.
Hot water can contain more dissolved salt.
Salt water is more dense than fresh water.
The salt gradient layer acts as a thermal barrier inhibiting convection.
The hot water can then be siphoned off and passed through an organic Rankine cycle engine.
81
What are the 3 layers of a salt gradient solar pond? How do they vary in salinity?
1. The surface layer (fresh water)
2. The non-convective layer (medium salinity)
3. The storage layer (saturated salt layer)
82
Are larger or smaller solar ponds more efficient?
Larger - more water to be heated & utilised = more solar energy absorbed.
83
Do larger or smaller solar ponds suffer from overshadowing of the storage (saturated salt) layer?
Smaller - by the pond walls.
84
What are the advantages & disadvantages of adding insulation to solar ponds?
Advantages - Maximum thermal gain/minimise heat loss and maintain heat in storage layer.
Disadvantages - insulation layer causes loss of incident solar radiation. Could be high cost relative to the benefits.
85
What are the advantages of solar ponds?
1. Simple to manufacture
2. Low construction costs
3. Salt water ponds can produce salt crystals as a by product
4. The pond can act as both thermal collector and thermal storage.
5. Solar radiation to heat the water is free
6. Reduced emissions in comparison to conventional fuel systems.
86
What are the disadvantages of solar ponds?
1. Evaporated water needs to be continually replaced.
2. Addition of a Rankine engine to generate electricity will increase costs (usually only on salt water systems)
3. Risk of transfer fluid leaks.
4. Salt crystals have to be removed from salt water ponds regularly
5. Low efficiency due to relatively low temperature differences that can be achieved.
87
How does ocean thermal energy conversion work?
By utilising the temperature gradient generated by sunlight heating up the surface water.
This 24°C temperature difference can be used to drive heat engines.
Warm water from the surface is passed through a heat exchanger containing a transfer fluid, such as NH3.
The evaporated transfer fluid then drives a gas turbine and generator to produce power.
The transfer fluid is recycled in a second heat exchanger where it is re-condensed using cold deep sea water. This is then pumped back into the evaporator.
88
Where on earth is the ocean temperature gradient the largest?
In the tropics.
89
What is macrofouling & microfouling & how does it impact ocean thermal energy conversion? How can this be reduced?
Macrofouling - growth of barnacles and seaweed
Microfouling - bacterial and algal growth
Both grow on the heat exchangers resulting in lower efficiency.
Fouling can be reduced by increasing the flow velocity (macrofouling) and adding chlorine or ozone for a limited period per day (microfouling).
90
What is the practical set up of a ocean thermal plant?
Most plants are sea based (less piping required). Flexible cold water pipes prevents damage during storms and they are easier to transport, construct and assemble.
Sea based plants must endure the harsh marine environment and may have stability problems.
Under water thrust jets are used to provide stability and limit drift.
Partially submerging the plant provides additional stability and protection from waves.
The offices, living quarters and deck cranes are located above the water surface.
Dividing the plant into separate sections allows maintenance to be done while the bulk of the plant is still operational.
91
What are the advantages of ocean thermal energy conversion?
1. Increases ocean biotic populations.
2. Can produce fresh water
3. Can produce fuels
4. Can produce chemical starting materials
5. Fuel used to heat the transfer fluid is free.
6. Reduced emissions in comparison to conventional systems.
92
What are the disadvantages of ocean thermal energy conversion?
1. High construction costs.
2. High maintenance costs.
3. Difficult to deploy, technical expertise required
4. Deep waters required with relatively large temperature difference from the bottom to surface.
5. Very low efficiency.
6. Erosion/fouling may take place.
