Thursday, June 9, 2016

Cost of solar power (63)

The conservative side of politics in Australia has a goodly share of climate change deniers and fossil fuel proponents.  So it’s a delicious irony that the former leader of the Liberal Party (that’s the conservatives!), John Hewson, has now emerged as the Chairman of Solastor, a company that proposes to build a large solar thermal power station near Port Augusta.  RenewEconomy has details of the proposal (here, here and here).

I’ve written about the costs of a CST plant at Port Augusta before.  See here for details.

The Solastor proposal involves a modular heliostat/tower approach.  Each module will have a 24 m tower on which is sited a 10 tonne graphite block for thermal storage.  The footprint of the mirror field is 65 m × 35 m, for a total area of 2,300 m2.  We are told there will be approximately 100 heliostats per module, and I’m estimating the mirror area per module to be about 1,000 m2.

The thermal collection system then drives a conventional steam generator.  We are told only that the working steam temperature is 400°C and the storage temperature in graphite is 800°C.  We don’t know whether the condenser is water or air-cooled.

So this proposal is unconventional only in the sense of graphite as the storage medium.  Vast Solar are also aiming at a modular approach.

Before estimating the Levelised Cost of Electricity (LCOE) for the Solastor approach, let’s make a few back-of-envelope calculations about the performance using figures provided by RenewEconomy and Solastor.

Thermal collection:  Let me assume the optical efficiency of the field is 80%, the efficiency of the receiver is 95%, the mirror field area is 1,000 m2 and the DNI is 6.5 kWh per m2 per day.  Then the heat collected per module per day is 6.5 × 1,000 × 0.80 × 0.95 = 4,940 kWhth.  That agrees remarkably well with the figure given by Solastor in their slideshow presentation.

Thermal storage: The specific heat capacity of graphite is 0.71 kJ/kg.°C.  If the storage temperature range is 500°C, then the heat stored by a 10,000 kg graphite block is 10,000 × 500 × 0.71 kJth = 3,550 MJth = 0.986 MWhth.  That’s only about 1/3 of the thermal storage claimed by Solastor in their slideshow presentation.  I’m concerned that the thermal storage is under-specified for the requirements.

Solar multiple:  We are told in the RenewEconomy reports that Solastor proposes a 170 MW plant with 1,700 modules.  My estimate of the instantaneous collection capability of a 1,700 module system at 1 kW DNI per m2 is 1 × 1,700 × 1,000 × 0.80 × 0.95 = 1,292,000 kWth.  At a thermal to electric conversion efficiency of 33%, the possible power output would be 426 MWe, so the solar multiple of the proposed system is 426/170 = 2.5.

Cost and annual output:  According to figures provided by RenewEconomy, the 170 MW system to be completed in 2018 would cost AUD 1,200 million and have annual output 1,229,000 MWhe.  The Capacity Factor would be 1,229,000 / (365 × 24 × 170) = 0.83, which seems way too high to me (I would expect a figure of about 0.60), but let’s not quibble for the moment.

Let me now estimate the LCOE for the Solastor proposal using my standard assumptions:

  • there is no inflation,
  • taxation implications are neglected,
  • projects are funded entirely by debt,
  • all projects have the same interest rate (8%) and payback period (25 years), which means that the required rate of capital return is 9.4%,
  • all projects have the same annual maintenance and operating costs (2% of the total project cost), and
  • government subsidies are neglected.
For further commentary on my LCOE methodology, see posts on Real cost of coal-fired power, LEC – the accountant’s view, Cost of solar power (10) and (especially) Yet more on LEC.

Note that I am now using annual maintenance costs of 2% of capital cost rather than 3% as in posts during 2011. 

The results are as follows:
Cost per peak Watt              AUD 7.06/Wp
LCOE                                     AUD 112/MWh

The components of the LCOE are:
Capital           {0.094 × 1.2 × 109}/{1.229 × 106 MWh} = AUD 92/MWh
O&M              {0.020 × 1.2 × 109}/{1.229 × 106 MWh} = AUD 20/MWh


My LCOE estimates for comparable CST plants are Cerro Dominador (USD 121/MWh), the previous Alinta proposal for Port Augusta (AUD 218/MWh) and Atacama 1 (USD 149/MWh).   The Solastor LCOE would certainly be competitive with other CST plants provided Solastor could deliver on their performance and cost estimates.  However the team behind Solastor are not as experienced as other world-class CST players such as SolarReserve, so Solastor’s claims must be regarded with some suspicion until further details emerge.

This graphic has further details of the 63 LCOE estimates I have compiled.