Your method is fine. A key question is the price of power in...

Your method is fine.

A key question is the price of power in 2015. By then carbon credit pricing will have been introduced as well as the natural CPI effects over eight years. The price that a geothermal utility can charge is therefore between that which a competitor can charge for base load and the peak load price. While we always talk of the virtue of geothermal being able to supply long term base load it is selling into peak load that can bring the significant returns. In principle, for increased power, GDY just needs to turn on the brine flow tap a few turns, although the design flow rate will have an optimal value. How rapidly can changes in heat flow pass through the two heat exchangers? This will determine the participation in peak load supply.

Base load charge (new start coal) may therefore be
~ (45 + 15) x 1.03^7 = $74 /MWh

Peak load pricing would be much higher but on a tight duty cycle.
If GDY is able to supply into both markets the approx mean price in 2015 could be $80-90 /MWh.

It is not altogether clear whether the proposed 500 MW is installed capacity or generating capacity. GDY probably don't know yet either. Initially there will be greater downtime due to start up changes, testing, and adjustments but in steady state the generating cap might be 90% of installed cap.

Hence a figure for GDY gross revenue could be,

$ 0.90 x 500 x 24 x 365 x 76
= $300 M

Now you will ask the hard questions, what are the costs of generation, and the cost of loans?
Note also that 500 MW is merely an intermediate goal in the ramp up to perhaps 10 000 MW capacity.

Juke