1. Impact of Energy Storage on Solar PV Grid Parity
(This article was written by Schalk Cloete and published on The Energy Collective)
Proponents of intermittent renewable energy such as solar PV and wind often claim that these
energy sources will reach parity with standard grid power in the near future. As discussed in
a previous article, however, this is a highly misleading claim, primarily because intermittent
and non-dispatchable renewable energy is worth much less per kWh than steady and
dispatchable baseline power.
In order to illustrate the implications of this distinction, the aforementioned article
valued intermittent PV similarly to unrefined coal. The central assumption underlying this
way of thinking is that the costs associated with energy storage (which is required to make
PV useful to society at higher penetration levels) are comparable to the costs associated with
thermal power plants (which are required to make coal and gas useful to society at higher
penetration levels). Under this assumption, solar PV turned out to still be about one order of
magnitude more expensive than coal power.
Naturally, this is a fairly crude assumption and accurate calculation of the real grid parity
target for solar PV will be much more complex. This article will discuss the most important
complexity: the fact that the costs associated with energy storage of intermittent renewables
will be a strong function of the level of penetration into the local electricity grid.
The cost of storage
Under the assumption that the costs associated with storage are similar to the costs associated
with thermal power generation, storage would increase costs roughly by a factor of 4 (as is
the case for coal at $100/ton and coal-fired electricity at $0.06/kWh). However, this cost
increase will be a substantial over-estimate at low penetration levels where almost no storage
is necessary and a substantial under-estimate at high penetration levels where most renewable
energy generated will have to be cycled through some form of storage.
The graph below illustrates the price at which solar PV reaches parity with coal for five
different storage cost scenarios assuming a coal price of $100/ton, a 30 year panel lifetime
and a 5% discount rate on gradually released PV electricity.

2. The most important comment to be made about this graph is that we will move downwards
with increased PV penetration. I am fairly confident that, for most locations, we will reach
the light blue line at the bottom long before intermittent renewables come close to supplying
100% of our electricity. The exact penetrations at which each of the lines on the graph will be
crossed is much more uncertain though. I will give some rough estimates in this article, but
would welcome any corrections by experts on this site.
Negligible storage costs (<1% penetration)
Initially, when solar contributes less than about 1% of electricity, the intermittency will be
essentially negligible. As the blue line shows, current utility scale installed PV prices
(~$2/Wp) are already close to parity with coal in the most ideal locations (highest PV
capacity factors) under this assumption. However, this first percent of solar PV penetration is
the only region where the standard grid parity mantra of renewable energy advocates is
relevant.
Storage costs half of power plant costs (10-20% penetration)
As we move up to a 10-20% penetration of intermittent renewables, we also move down to
the red line in the graph. Under this scenario, solar PV (and wind) starts to rely significantly
on the energy storage implicit in fossil fuels. Standard power plants then have to be operated
at lower capacity factors and at lower efficiencies due to more ramping and more spinning
reserve.
One recent study for wind power calculated that costs of keeping backup fossil plants
operating at lower capacity factors and efficiency (together with some added transmission
costs) would increase the real cost of wind to triple the price of new gas and 1.5 times the
price of new coal in the US. This represents a doubling of the standard costs calculated when
the intermittent and non-dispatchable nature of wind energy is simply ignored.

3. Storage costs equal to power plant costs (20-40% penetration in selected
regions)
As we move beyond a 20% penetration of intermittent renewables, specialized energy storage
becomes necessary. According to EIA estimates, the most feasible option; pumped hydro
storage, will cost about twice as much as a coal plant per watt. It will, however, lose only
about half the energy lost by the coal plant in the energy conversion process. It can therefore
be estimated that pumped hydro storage will inflate solar PV prices roughly by the same
factor as a thermal power plant inflates the price of coal.
Even though the installed PV price of roughly $0.3/Wp required by this scenario seems
highly unlikely ever to materialize, it should be noted that regions with abundant natural
hydro capacity could potentially achieve these penetration levels of intermittent renewables at
much more affordable prices. Denmark's wind backed up by hydro from Sweden and Norway
is one such example. Very few regions on earth are suited for this kind of arrangement
though.
Storage costs double power plant costs (20-40% penetration in most regions)
Pumped hydro is only available in certain (relatively rare) topographies. Thus, for most cases,
a day or two of battery storage will be most practical. Despite lots of noise from battery
optimists, the 150-year old lead-acid battery is still the cheapest option we have for this
purpose, but suffers from drawbacks such as short lifetimes (especially at deeper discharge
rates) and relatively low efficiencies (about 20% of energy cycled through the battery is lost).
Lithium-ion batteries reduce these problems, but are also more expensive. One case
study found that a lead acid battery and a lithium-ion battery could store energy for about
$0.34 and $0.40 per KWh over their respective lifetimes. This cost (which must be added to
the cost of renewables) is much greater than fossil fuel power even by itself. To better link
this to the graph above, consider that most suppliers will sell you about $4 of batteries per
watt of solar PV for protection against blackouts (example) where the battery warranty period
is only about half that of the panels.
It should also be mentioned that, in the hypothetical scenario of very cheap solar PV and
relatively expensive storage, it could be more economical to simply build a large
overcapacity of intermittent renewables and spill a large portion of the power produced. In
the graph above, this will reduce the capacity factor of the installation, but could create a
transfer from the purple to the green line.
Storage costs quadruple power plant costs (>40% penetration)
Finally, the light blue line right at the bottom comes into play when one starts thinking about
longer term energy storage to compensate for longer cloudy (or wind-still) periods or even for
slow seasonal variations. This line (which really is a matter of complete impossibility for
intermittent renewables) is especially applicable to regions with long cloudy spells and
seasonal mismatches (e.g. solar PV in Germany). On the flipside, however, it is also much
less applicable to regions with very reliable renewable energy resources that are well aligned
with seasonal demand (e.g. solar PV or solar thermal in desert areas).

4. Chemical storage is probably the only viable option for such longer-term storage
requirements with hydrogen normally being the first option that comes to mind. A Spanish
study found that hydrogen from combined wind and solar projects would cost about €25/kg
which translates to about $0.90/kWh of hydrogen internal energy. Converting this stored
energy back to electricity at a later time will inflate the price by another factor of 3 (similar to
natural gas power plants), bringing the total cost up to $2.70/kWh - about 50 times more
expensive than conventional power. Other forms of chemical storage might be more
economical, but it will be very difficult to rise above that light blue line.
Conclusion
The previous article stated that current solar PV technology is still about one order of
magnitude more expensive than coal. Based on the above analysis, it can be stated that this
will be the case at roughly 20-40% penetration of solar and wind into our electricity networks
(about 8-16% of total energy), beyond which the prospects for PV (and wind) will rapidly
deteriorate. This is a good example of the law of receding horizons discussed earlier.
However, the cost of storage is not the only influential variable in determining the real grid
parity target for solar PV. The next article will therefore investigate four additional factors:
the coal price, the PV panel lifetime, the PV discount rate and a CO2 price.
Schalk Cloete
I am a research scientist searching for the objective reality about the longer-term sustainability of
industrialized human civilization on planet Earth. Issues surrounding energy and climate are of
central importance in this sustainability picture and I therefore seek to learn more from the Energy
Collective community. My current research focus is on second generation CO2 capture processes
because these systems will be highly suited to the belated climate action that will probably start
once climate change really starts to affect the average member of the democratic electorate.