Short Questions Flashcards
Is storage more important for renewable or fossil fuelled power stations?
Renewable power stations.
These are intermittent sources where it is more difficult to match supply and demand.
What is the most widely used energy storage technique?
Hydro (water held by dams at varying altitude).
List four ways of storing energy. Give their advantages and disadvantages?
Hydro - High power. BUT needs real estate.
Batteries - Easy, rapid response, reasonable power density. BUT expensive and limited life.
Capacitor Bank - Easy, very rapid response, high power density. BUT low energy density.
Hydrogen/Methane - Stored for a long time (and no degradation). BUT needs production method heat engine and infrastructure.
How do geothermal stations and ground source heat pumps differ?
Geothermal uses heat from the ground. The temp removed from the ground is equal to or less than that of the ground.
Ground source heat pumps take low grade energy from the ground and increases its temp through a reverse heat engine. The energy produced is greater than the average ground temp.
Which releases more energy, the fusion of Deuterium and Tritium or the fission of U235?
Fusion releases far more energy.
How is Tritium obtained?
Transmutation of Lithium by bombarding it with neutrons. This creates Helium and Tritium.
What are the units of the Sievert?
J/kg
How much background radiation does a human receive each year?
2mSv/year
Define the cross sectional area for a specific nuclear reaction of a single nucleus.
The number of specific reactions per second divided by the incident neutron flux (number of neutrons per m2) for one single nucleus.
Units = m2
Define the macro cross sectional area for a specific nuclear reaction per unit volume.
The number of specific reactions per second divided by the incident neutron flux (number of neutrons per m2) for a unit volume of material containing N nuclei.
Units = 1/m
What is the typical vertical gradient in the first 10km of the Earth’s crust?
20-30 degrees Celsius / km
What are the typical losses for biomass energy storage on the surface of the Earth?
1/3 falls during growing times 1/5 falls on leaves 1/5 is reflected 1/2 has correct wavelengths 1/3 converted into stored energy 2/5 of stored energy is used by the plants
0.5% is stored in biomass energy.
Rank the following fuels in terms of energy density.
- Natural gas
- Oil
- Dried wood
- Coal
- Fresh cut grass
- Natural gas
- Oil.
- Coal
- Dried wood
- Fresh cut grass
What is the maximum energy available when a hydraulically sealed lagoon is full of water?
(pgAH^2)/2
What is the relationship between the power available in ocean waves and the wave period?
P is proportional to T
Give the typical energy content of 1kg of diesel fuel.
10kWh
Which has a higher energy density, methane or petrol?
Methane. It has a higher ratio of hydrogen:carbon atoms.
The oxidation of hydrogen produces more energy per kg than the oxidation of carbon.
Identify the most plentiful ways in which carbon presents itself on Earth.
- Mineral form (chalk)
- Dissolved in water (oceans)
- Living matter (algae, plants and animals)
- CO2 in the atmosphere
- Fuel (coal, hydrocarbons, etc…)
Is a steam engine an internal or external combustion engine?
External.
The working fluid is held internally, but it is heated by combustion that occurs externally in another chamber.
Give the formula for Carnot efficiency.
n = (Th - Tc)/Th… this is at zero power
n = 1 - (Tc/Th)… this is at max power
What kind of reactor is being built at Hinkley Point C?
Thermal Pressurised Water Reactor
Which has better thermal efficiency, gas cooled or water cooled reactors?
Gas cooled.
Water cooled reactors have an upper operating temperature set by the critical point of water, which is relatively low (374C). Gas cooled don’t suffer from this and the operating temp is set by the mechanical integrity of the fuel canning material which is much higher (600-900C).
What is co-generation?
Co-generation uses the waste heat from power generation as a valuable input for a process that requires heat.
Optimised co-generation uses thermal energy in a manner that produces the optimum mix of power and heat to satisfy requirements.
Is geothermal a renewable energy?
Maybe. The reason some don’t regard it to be is because the rate of extraction is usually faster than regeneration, therefore it is unstable in the medium to long-term.
Give the available power per unit area in a fluid stream.
0.5pv^3
What is the energy available in ocean waves (with amplitude height H) proportional to?
H^2
What is the power from a hydro power unit (with velocity V) proportional to?
V^3
What is the power from a hydro power unit (driven by a head, H) proportional to?
H^(3/2)
Which type of silicon based photovoltaic device gives the highest voltage?
Monocrystalline
Why is geothermal not more widely adopted?
It requires the appropriate geological structure with significant thermal resources close to the surface.
It needs lots of infrastructure (more than fossil power stations and wind turbines).
What gas does fracking retrieve?
Methane
Which has a larger volume for the same power output, gas cooled or water cooled thermal nuclear reactors?
Gas cooled.
The heat transfer rates are much lower in single phase gaseous systems than water/steam systems.
What was the typical ppm of CO2 in the atmosphere in 2016?
400ppm
What was the estimated concentration of CO2 in the atmosphere in 1700?
300ppm
What is the ratio of the tidal effect of the Sun to the Moon?
1:2
Give the formula to find the barycentre of the Earth and the Moon.
(XMe)/(Mm + Me)
Why does the same side of the Moon always face the Earth?
The Moon spins on its axis at the same rate that the Moon orbits the Earth.
The reason for this is believed to be due to the gravitation drag force that seeks to synchronise the rotation of the Earth/Moon with the rotation about themselves.
Why is the hot temp of the working fluid in an externally heated thermal engine lower than the maximum temperature of the heat source, in order for the engine to produce the maximum possible mechanical power?
To ensure heat flow, a driving temperature is needed. This ensures . that a significant amount of heat flows to the working fluid. This temp difference needs to be significant, however there is still the need to ensure that the difference between the working . fluid’s hot and cold temp is significant.
What happens if a PV panel is connected to a rechargeable battery but there is no light falling on the panel?
The battery will discharge through the PV panel.
When the temp of a PV panel is increased, what happens to the current, voltage and power?
Current - Increases due to the increased electron mobility and density.
Voltage - Decreases due to the decreased gradient across the p-n and n-p junctions.
Power - Decreases because V decreases more than I increases.
In fracking, what is a proppant?
The material that holds open the crack in the ground. They are usually sand-like particles.
Which factors limit the size of wind turbines?
- Mechanical limits
- Tip speed creates drag effects
- Noise
- Visual impacts
How does the ideal power of a turbine vary with the air flow rate going through it?
Q^3
Why do wind turbines and hydro-machines share the same relationship to flow rate?
Energy is proportional to Q^2. Power is energy per unit time, therefore it is proportional to Q^3.
Why do deep and shallow waves have different speeds?
In shallow water, the waves are influenced by depth due to the dampening effects . of the sea bed slowing the water down (g x depth)^0.5.
In deep water, the effect of the sea bed is negligible and speed is more related to the wave length of the waves. For very deep systems, the wave speed is proportional to ((g/2pi) x wave length)^0.5.
