Practical Limits of Renewable Energy

Often I’ll hear people who want to put PV on every rooftop in my sunny city, or who don’t expect any new coal power plants to be built ever again. Neither situation is very realistic for a number of reasons. The root issue is how much renewable resources can we rely on, and how do those numbers change with future advances in technology.

Solar PV is often looked to as a simple solution – put the panels on your roof and you can generate between 50 and 100% of your power requirements over the year. It simplifies a number of things – like treating the grid like a gigantic battery, where energy generated that isn’t used locally is sent out to the grid and when the panels aren’t producing enough, to pull off the grid to make up the difference. The issue is that if you have a large PV installation designed to generate power, and the sun goes behind a cloud and your output drops by 50%, the power demand doesn’t instantly drop by that much, so you have to make it up elsewhere. This “elsewhere” is usually natural gas, but natural gas can only spin up so fast – from 50% to 90% in 6-7 minutes, by then the sun might be back out from the cloud and production rates are already returned to normal. This limits the amount of large free field PV that can be installed – its not so much about cloudy and rainy days since you can forecast and plan for additional resources to be run that day, its the intermittent cloudy days where your output varies all day. Geographically distributing the generation can help but building large facilities far away from each other can hurt the total cost effectiveness of the project. I would put the overall piece of the generation pie at 2.5% of total energy generation.

The proliferation of roof-top systems could double these numbers if utilities can get a better grip on the demand side – being able to turn off non-essential grid demands like plug-in cars, and being able to ramp power quicker when PV generation quickly drops (draw on energy storage – either batteries, thermal or kinetic). This is where the smart grid is absolutely critical. Also, predictive systems could also cut consumption – your air conditioning unit sees that the sun is behind a cloud so it can reduce energy consumed because your house isn’t exposed to direct sunlight and wont heat up as quickly as it would have otherwise, instead of just turning on when the internal temperature creeps up.

Solar Thermal is an easier approach from the grid operator’s perspective. Since solar thermal is still driven by hot liquids and turbines, it has a higher generational “inertia” than solar PV does, that is to say the output doesn’t change as quickly as PV does when the sun goes behind a cloud. This would allow for other natural gas turbines to speed up and add generation output. Likewise, its possible to build storage in the form of a large molten salt tank that could be drawn on when the sun goes behind a cloud, as well as after the sun sets. A 100MW facility that had 50MWh thermal storage could provide for up to 30 minutes without any sun, or a proportionally longer time during partial sun exposure. Thermal storage is still far preferable than batteries for the foreseeable future. It wont be until there are large quantities of partially used batteries to be recycled for other uses (old PHEV batteries that are retired after 10 years) that battery storage will become more useful. This technology has a substantially larger piece of the pie – 12.5% of total energy generation.

Wind has an upper limited of around 20% with today’s “dumb” grid. With the creation of a smart grid and devices that can defer their energy consumption to cheaper times or whenever there is extra capacity, wind will be able to grow beyond the 20% barrier. Beyond that, other large scale storage mechanisms like pumped hydro would allow for wind energy to be converted into kinetic energy in the form of water stored in a reservoir, however this is a fairly expensive exercise – instead of just the cost of the wind turbine and transmission lines, the additional pump storage system costs have to be factored into the total cost of wind power. Wind could account for about 20% of energy generated.

Geothermal technologies aren’t really limited by reliability, since they substitute for baseload power (coal, nuclear). Rather its limited by the land suitable for geothermal wells, mostly in the western parts of the US. Standard geothermal requires pumping hot water (at least 300° F) from underground and letting it turn into steam to turn turbines and then either releasing the steam into the atmosphere or attempting to recapture it and put it back into the ground. Even lower temperature geothermal, which can use water down to about 165°F, still is limited in areas it can be used, though it provides a larger footprint in terms of how much energy we can extract. Enhanced Geothermal involves fining hot rock and then artificially recharging the aquifer with water to be able to extract that hot water elsewhere and use it for generation purposes. Combined, these technologies would only account for about 5-7% of installed rated power, but because of their high capacity factor provide around 15% of our energy generation portfolio.

Biomass, using waste from humans (trash) as well as marginal plant products (the famous example is  switchgrass) to produce power is also an option, however it is further behind from a technological standpoint that solar or wind. However it could function as a baseload power plant, which is highly desirable from a CO2 reduction standpoint since it can replace coal power. The biomass is gassified and then put through a turbine to generate electricity. I don’t see this ramping up soon, the projects are currently very small and no where the size for what a utility would require for a generation facility (100MW or larger).

Combined, all of these green technologies can account for approximately 50% of our energy generation needs. When combined with nuclear and hydro, we could have 80% of our energy from non-fossil fuel sources. However its not going to be that easy to shut off coal – only geothermal and biomass have sufficiently high enough capacity factors to replace coal. Combined, they’re likely to only be able to displace about 30% of coal’s energy output.

To eliminate coal will require a very large, geographically diverse wind power footprint – with a larger capacity than I think the current grid can support. To replace 70% of coal with wind power, it would take approximately 450GW of installed capacity – higher than most estimates have predicted.

Any final drive to remove the last few coal plants by 2030 will require replacing them with baseload natural gas power plants, which are subject to the volatile prices of natural gas (in 2008, natural gas was $9/MMBTU, now its approximately $3.50/MMBTU, I have no idea where it’ll be next year).

We should always work for a cleaner environment, but ignore the reality of technology at this point is not advisable. Where alternatives exist (geothermal in the west, solar in the southwest, wind in the midwest) we should shun coal power, but we don’t have the option to turn it off completely. Cap and Trade could push us further in that direction, but it shouldn’t have the end goal of eliminating coal, rather seeking to reduce CO2 – which would have the effect of preventing coal power expansion and forcing coal burning facilities to do what they can (including building newer, comparatively cleaner) to clean up.