Nuclear + storage

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Close-coupled nuclear generation & compressed air energy storage (CCNG-CAES)

This case study – original work © Fluke – illustrates the scale of operation needed to provide 1.1 GW (one gigawatt) of electricity on a twelve hours on, twelve hours off cycle. This could be used for peak shaving renewable energy or balancing periods of low and high demand.

The base case, without storage, is a 2.0 GW (thermal) output nuclear power station producing electricity via steam turbines at a thermodynamic efficiency of one-third (33⅓ %) 24-hours per day, so 0.67 GW (electrical) continuously.

The study case is an identical power station, located adjacent to a large underground void.

It is feasible to use mechanical energy to compress air and place it into the void, which acts as a pressure vessel. The compression takes place in four stages, each compressor being directly driven from the output shaft of a steam turbine. SIX such arrangements in parallel are required to consume the entire thermal steam output and under this operation no electricity is generated.

At other times air is drawn from the store, heated (and reheated) using 43% of the steam output of the nuclear power station as it expands through four stages of air turbine driving generators. The balance of steam 57% is used to generate more electricity from three steam turbines. Again SIX of these lines are required to consume the entire thermal output and the power generation per line is 183 MW so 1.1 GW gross

So comparing cases, without storage the nuclear power station delivers 16 GW-h electricity continuously over a 24-hour period while the CCNG-CAES generates 13.2 GW-h for 12 hours and nothing for the other 12. The analogy is inexact but the system is in effect a 13.2 GW-h battery with a round trip efficiency of 82.5%

It is also informative to compare this with SSE’s Coire Glas pumped hydroelectric scheme – 30 GW-h storage with 1.5 GW input/output

Void requirement

Each of six lines handles 850 tonnes per hour (850,000 kg) of air with the void pressure cycling between 5 MPa (50 bar) ‘discharged’ and 6 MPa (60 bar) ‘charged’. Density of air at 35°C at the two pressures is 56.85 kg/m³ and 68.21 kg/m³ respectively; difference is 11.37 kg/m³. One line for one hour therefore requires 74,787 cubic metres, multiplying by 6 lines and by 12 hours gives a total requirement of 5,385,000 m³.

For example the void space at Veolia’s Minosus facility at Middlewich, Cheshire which is used for hazardous waste storage was 23 million cubic metres (2006). Ongoing salt production of 1 million tonnes per year produces new void of about ½ million m³ a year.

Compression stage

Fresh air intake (design conditions 20°C, 1 bar) is compressed in four stages with intermediate and final pressures respectively 2.8 bar, 7.8 bar, 22 bar and 60 bar (6 MPa); the pressure ratio at each stage is ~2.8. Compression is carried out with an assumed isentropic efficiency of 0.85 and the procedure can conveniently be calculated by spreadsheet, using an equation of state for air that provides temperature as a function of (pressure, enthalpy) or (pressure, entropy); enthalpy function of (temperature, pressure) or (pressure, entropy) and entropy as a function of (temperature, pressure). For this study the equation of state used was the NIST Refprop 9.0 program. The following table illustrates the basic equations used to construct the spreadsheet

State Description T/C P/MPa h/kJ kg-1 s/kJ kg-1 C-1
1 Initial condition T1=20 P1=0.1 h1=fn(T1,P1) s1=fn(T1,P1)
2 Isentropic compression T2=fn(P2,s2) P2=2.8*P1 h2=fn(P2,s2) s2=s1
2' Compression(IE=0.85) T2'=fn(P2',h2') P2'=P2 h2'=h1+(h2-h1)/0.85 s2'=fn(T2',P2')
3 Intercooling T3=35 P3=P2' h3=fn(T3,P3) s3=fn(T3,P3)
4 Isentropic compression Repeat stages 2-2'-3
4' Compression(IE=0.85)
5 Intercooling