Solar re-powering Georgia's Plant Bowen in sunny Spain

Solar re-powering Georgia Power's Plant Bowen to 3.2 GWe (75% capacity factor) in Andalucia's climate would require a storage/boiler compartment about 60% as tall as one of its cooling towers—and 130% its diameter. The central optics would be about 110% the height of its smokestacks.

In Andalucia, Spain, the location of the Gemasolar plant, only 15 hours of thermal storage (plus whatever solar multiple, or thermal down-rating, Gemasolar is using) is needed to achieve an annual capacity factor of 75% in a solar thermal generating plant.

Plant Bowen (3.2 GWe and 82% capacity factor) in Euharlee, GA, USA, is the largest coal-fired plant in the United States, and the country's largest point source of CO₂ pollution. This post contemplates what Plant Bowen would look like if re-powered to run on the sun in Andalucia's sunny climate. Of course, it would be more interesting to see what Plant Bowen would look like solar-powered up to its own 82% capacity factor in northwest Georgia's own, somewhat less sunny, climate, but that would involve a detailed simulation. By figuratively moving Bowen to Spain, and adopting Gemasolar's 75% capacity factor, we can just steal data.

Quoting an earlier post:

Here are some statistics for Gemasolar gleaned from the National Renewable Energy Laboratory's site. 

Projected annual output: 110,000 MWhr/yr = 12.6 MWe  annual average.
Rated output  (calculated from the claimed 75% capacity factor): 16.7 MWe rated.
Output per mirror area (304,750 m2) :
       41 We/m2 annual average,
       55 We/m2 rated.
Land yield (1,950,000 m2; mirror/land ratio = 0.156):
        6.4 We/m2-land annual average,
        8.6 We/m2-land rated 

The 15 hours of storage based on 40% thermal efficiency is:
15 hrs * 3600 s/hr * 8.6 We/m2-land * 1/0.40 = 1.2 E6 Jthermal/m2-land 

A plant with telescopic heliostats and glass-melt storage would have some advantages over Gemasolar. Telescopic heliostats can be packed much more closely, increasing the mirror/land ratio to around 0.70, thus increasing land yield about 4.5 times that of Gemasolar. Also, because the glass melt transfers its heat to hotter steam (608°C vs. 565°C) the steam cycle efficiency can be greater, about 46% thermal efficiency as compared with 40%, a factor of 1.15 . 

So here are the Gemasolar statistics if it were rebuilt on the same plot of land with telescopic heliostats and glass-melt storage: 

Projected annual output: 110,000 MWhr/yr * 4.5 * 1.15 = 65 MWe  annual average.
Rated output  (calculated from the claimed 75% capacity factor): 87 MWe rated.

Output per mirror area (304,750 m2 * 4.5 = 1,370,000 m2) :
       41 We/m2 * 1.15 = 47 We/m2 annual average,
       55 We/m2 * 1.15 = 63 We/m2 rated. 

Land yield (1,950,000 m2; mirror/land ratio = 0.156):
        6.4 We/m2 * 4.5 * 1.15 = 33 We/m2-land annual average,
        8.6 We/m2 * 4.5 * 1.15 = 45 We/m2-land rated. 

The 15 hours of storage for the rebuilt plant becomes:
15 hrs * 3600 s/hr * 45 We/m2-land * 1/0.46 = 5.3 E6 Jthermal/m2-land 

Plant Bowen is a plant belonging to Georgia Power in Euharlee, Georgia. It is the largest coal-fired plant in the USA. It has four 800 MWe units, giving an aggregate rating of about 3.2 GWe. A telescopic heliostat / glass-melt power plant in Andalucia with 15 hours of thermal storage, having the same rated output of Plant Bowen, would occupy: 

3.2 GWe-rated / 45 We/m2-land rated = 71 E6 m2,
which is equivalent to a circle 4.8 km in radius.

The height of the central optics will be about 1/14 the field radius, or 340 m, or about 11% taller than Plant Bowen's 305 m smokestacks.

The heat flow to drive rated output is

3.2 GWe-rated * 1/0.46 = 7.0 GWthermal-rated

It remains to calculate storage and boiler dimensions. A boiler's water tube walls usually receive a thermal flux of around 250 kw/m2. Taking that value as a given, the total area of the water tube wall, S_B, will be

S_B = 7.0 GWthermal-rated / 250 kw/m2 = 28,000 m2.

The volumetric storage density in molten glass is

ΔT * 2300 kg/ m3 * 1231 J/kgK = ΔT * 2.8 E6 J/m3-K

Thermal storage needed for 15 hours of rated output is

7.0 GWthermal-rated * 15 hr * 3600 s/hr = 3.8 E14 J

So thermal storage volume V is

V = 1.35 E8/ΔT  m3

For a hemisphere

V = 2/3 π R³

so,

R =  (3/2π V)^0.33 m

R = (3/2π * 1.35 E8/ΔT)^0.33 m

R = (6.4 E7/ΔT)^0.33 m

The height, H, of the water tube wall can be calculated from

2πR * H = S_B


H = S_B * 1/2πR = 28,000 m2 * 0.159 / R = 4,460/R m

Exploring these relations in a Numbers spreadsheet shows that T_empty = 1570 °C (1840 °K) gives a consistent solution with R = 63 m, H = 71, and the flux on the water wall tubes = 250 kw/m2. The glass temperature range from empty to full is just 230 °C. T_empty is approximately the temperature of a glass-making furnace, so it is fair to say that the thermal storage is accomplished by overheating a soda-lime glass-making furnace by about 230 °C.

By comparison, Plant Bowen has four cooling towers that are 47 m in radius and 116 m tall—so the volume of the storage/furnace compartment of a solar-fired Plant Bowen would be comparable in volume to one of its current cooling towers.