Dugga 1 Flashcards
“What is the efficiency of an electricity-to-electricity hydrogen storage system?
(provide calculations)”
eta_electrolyzer=0.7
eta_storage=0.98
eta_fuelcell=0.5
eta_tot=eta_electrolyzereta_storageeta_fuelcell=0.35
Give the three key properties of a hydrogen storage system (in which hydrogen is produced from electricity to supply a hydrogen demand) and explain how these properties influence its role in the energy system.
“1. High cost of electrolyzer and 2. low cost of hydrogen storage makes it act as a complementing
strategy and the electrolyser is operated all hours except at high net load when the storage supplies the load. Much lower storage cost for
large storage sizes makes it more applicable to large-scale applications.”
Give two properties of hydrogen as energy carrier which makes it better at managing temporal variations in VRE availability than geographical variations in VRE availability.
Hydrogen has low volumetric energy density and high diffusion capabilities which makes it difficult to transport.
Give three properties which define the flexibility of thermal generation.
Start-up time, start-up cost and minimum load level
a. List three differences between a thermal powerplant with a steam cycle compared to a thermal power plant with a gas cycle
b. Explain how these differences impact their respective role in the electricity system
a. Working media, cost structure, size, combustion (single-phase or multi-phase)
b. With single phase combustion and gas as working media the plant can be smaller and heated and cooled faster. This gives shorter start-up time and lower start-up cost and lower investment cost. However, the fuel deployed is expensive. This plant is thus preferably used for mid-merit or peaking operation. With multi-phase combustion and steam as working media the plant is larger and heated and cooled slower which gives high investment cost and high start-up cost. However, cheap fuels can be used. This plant is thus good for base load operation.
If thermal generation is operated flexibly the cost of operation increases for two main reasons, which?
- Start-up costs from fuel consumption during start-up and 2. increased maintenance cost and/or shorter lifetime due to thermal stress.
“Give one technical and one non-technical limitation on hydropower flexibility
(explain the reasons for the limitations).”
“Dynamic loading of turbine at deep part load (less than 60%)
Environmental court restrictions on minimum flow (see lec 8 for more options)”
A cost-minimizing actor is building two hydropower stations with storage in two different electricity systems; system A in which 10% of the electricity demand is supplied by wind power and 50 % by nuclear power and system B in which 40% of the demand is supplied by wind power and 20% by nuclear power. The new hydropower stations will supply the remaining 40%. Which hydropower station (the one in system A or B) will have the highest full load hours? Why?
“System A has highest full load hours since system B will be wanting more hydropower
capacity to compensate for wind variations.”
a. Give one technical and one non-technical limitation on hydropower flexibility (explain the reasons for the limitations).
b. State the hydropower storage equation and explain how the limitations in relate to the equation.
a. many options, e.g.Technical: Cannot operate the turbine on deep part load or overload. Then there is a risk that vortices create cavitation on the turbine baldes. Non-technical: Have to stay within environmental legislation on water level in reservoirs.
b. s(t+1)=s(t)+I(t)-g(t) Where s is the storage level of the hydropower reservoir at time t, I is the water inflow to the reservoir and g is electricity generation. Environmental legistaltion gives upper and lower values of s. Risk of cavitation give upper and lower limit on g.
In which three ways can electrification of industry provide flexibility in the electricity system?
flex in time, location and CO2 utilization
Give two ways in which a hydrogen-based steel production process can be designed to provide flexibility in the electricity system?
“1. Oversized electrolyser with H2-storage
2. Oversized electric arc furnace with HBI-storage”
“Give one example of how an electrification of industry can provide variation
management.”
Hydrogen can be deployed in the steel and refining industry. This hydrogen can be produced from water in electrolysis. The electricity consumption of the process can be flexible if there is a hydrogen storage available and the electrolyser is oversized. Electricity consumption will be avoided during high price hours providing complementing variation management.
“Explain why a heat pump greatly benefits from a thermal energy storage in a system with high levels of vRE.
What is the case for electric boilers?”
“The heat pump has high investment cost and is operating most of the time. With a heat storage the heat pump can avoid operation during high electricity prices while the storage meets the heat demand.
The EB has a low investment cost and is only used to act opportunistically during low net load events and therefore does not benefit as much from storage.”
Give three heat storage options and one main advantage and disadvantage for each option
- Tank heat stoarges: +fast discharging -expensive
- Pit storage: +low cost -requires large area
- Bore-hole storage: +low cost -slow discharging
Provide two explanations each for how thermal energy storage can help manage variations in
a) the district heating and
b) the electricity system?
a) the storage can be charged when heat demand is low using base load heat generation such as CHP or HP and discharged when heat demand is high to replace peaking units such as HOB.
b) the storage can be charged when by running the HP even if heat demand is low when electricity prices are low and then discharge the storage when electricicty prices are high and heat demand is high to avoid HP operation.
