Q2. Why is the optimum aspect ratio lower in the cold reservoir?
Due to the lower average gas density, pressure losses are more significant in 16 the cold reservoir and hence the optimum aspect ratio is lower and optimum particle size larger than for the 17 hot reservoir.
Q3. How many percent of losses are associated with the compressor and expander performance?
In the optimised designs, 24 losses associated with pressure drop and irreversible heat transfer in the stores are only a few percent, so 25 the success of PTES is likely to hinge upon compressor and expander performance.
Q4. What is the model used to quantify heat loss in packed beds?
The model used to quantify these losses is based on the well-established Schumann model of 31 heat transfer in packed beds [17], which assumes that the flow is one-dimensional and that heat transfer is 32 limited by surface effects (i.e., the internal thermal resistance of particles is negligible).
Q5. What are the main reasons for the increasing uptake of renewable sources of energy?
The finite nature of fossil fuel reserves together with a wide range of health and environmental concerns 2 arising from the release of combustion products have been acting as drivers for the increasing uptake of 3 renewable sources of energy, such as solar and wind [1].
Q6. What is the effect of a longer charge and discharge period on the energy stored?
Increasing Π 2 (i.e., longer charge and discharge period) obviously increases the energy stored per cycle, but this is at the 3 expense of lower efficiency.
Q7. Howes argues that there are optimum values for some design variables, whilst?
Parametric studies reveal that there are optimum values for some design 14 variables, whilst others either lead to a trade-off between efficiency and energy density or can be varied so 15 as to improve both these quantities together.
Q8. How are the equations governing heat transfer in the reservoirs integrated?
Heat exchangers, compressors and expanders are treated as steady flow 13 devices (in the time-averaged sense), but the equations governing heat transfer within the reservoirs are 14 integrated in time in order to track the hot and cold thermal fronts.
Q9. Why is the reduction in heat exchanger loss due to the avoidance of exit losses?
The associated reduction in heat exchanger loss is really due to 39 the avoidance of exit losses that occur when the thermal fronts emerge from the stores (these losses are passed 40 on to the heat exchangers, rather than being associated with the stores themselves).
Q10. How many GWh of storage capacity will be required over the next few decades?
In the UK, for example, it is estimated that over the 11 next few decades the integration of intermittent sources into the power infrastructure will require storage 12 capacities of the order of hundreds of GWh – an order of magnitude greater than current capacity [2].
Q11. What are the important factors in determining the merit of any electrical energy storage technology?
39 40 The important factors in determining the merit of any electrical energy storage technology are its round-trip 41 efficiency (i.e., the fraction of electrical energy input retrieved upon discharge) and its capital costs per MW 42 installed capacity and per MWh of storage.
Q12. Howes argues that heat 7 leakage for a 2 MW machine should be?
Based on estimates from 6 early prototypes and approximate (not fully non-dimensionalised) scaling, Howes [12] argues that heat 7 leakage for a 2 MW machine should be negligible, which according to Fig. 8 would reduce thermodynamic 8 losses by another 3.5%.
Q13. How does the efficiency of a reciprocating device differ from a CAES?
Predicted efficiencies 26 and storage densities obviously depend on the assumed loss factors; with an ‘optimistic’ set of parameters 27 that might be achievable with reciprocating devices, the thermodynamic round-trip efficiency could exceed 28 85% whilst the system simultaneously achieves an energy density almost an order of magnitude greater 29 than that for CAES.
Q14. What is the description of the thermodynamic aspects of pumped thermal electricity storage?
9 10A study of thermodynamic aspects of pumped thermal electricity storage (PTES) has been presented, based 12 on steady flow analysis of the compression and expansion devices coupled with a Schumann-style model of 13 the hot and cold thermal stores.
Q15. What is the power and storage capacity of a hypothetical PTES system?
The power and storage capacity given below are ‘nominal’ 39 values in the sense that they are the values that would be achieved in the absence of losses and in the 40 (hypothetical) case where the reservoirs can be fully charged.
Q16. Howes explains the sensitivity of the round trip efficiency of a reciprocating compressor?
For a 2 MW machine using an 5 induction motor-generator, electrical efficiencies of 97% (in each direction) are commonplace, but 6 mechanical losses for a custom-built reciprocating compressor-expander are less easy to estimate.