Grist recently discussed the new National Renewable Energy Laboratory (NREL) large wind study [pdf]. This study explores scenarios for supplying 20 percent to 30 percent of total electric energy consumption used by the eastern grid through wind power. (The eastern grid serves about 70 percent of the U.S. population.) Although it was not the main focus of the study, the NREL showed that in large-scale renewable scenarios the cost of transmission is much less than when only a small amount of renewable energy is used. For example, one often sees figure of $1,200 per KW for transmission costs for new wind. But if we look at transmission costs on page 39 of the NREL report, and MW generation cost on page 26 of the study, the results pencil out to around $160 to $200 in transmission and distribution costs per KW of capacity.
Although this is supported by really rigorous technical analysis in the study, some simple common sense combined with some knowledge of how the renewable industry works may help explain this.
Quite often short AC transmission lines are proposed to rescue stranded wind, essentially to bring power from a single wind farm to the grid. In many other cases transmission is suggested to support a handful of renewable generation facilities. In those circumstances, that line will be used at an extremely small percentage of capacity, even compared to normal transmission capacity use. (For very good reasons transmission lines are almost never used at anything close to capacity. Peak transmission is supposed to be for rare peak cases. I think something like 30 percent average capacity usage is not uncommon in long distance transmission.) Now a transmission system connected to a lot of renewabled in many different places will end up with a somewhat lower capacity utilization than one that is used for fossil fuel, hydro, and nuclear energy. But given diverse renewable sources the difference will not be nearly as great as with a renewable monoculture, or even a renewable scenario with limited diversity.
Look at it this way. A new current generation single large wind generator will operate at about 35 percent on average of nameplate capacity. But, that single generator will occasionally reach full nameplate capacity, or at least come very close. In a wind farm with hundreds of turbines this may never happen. When diverse wind farms in different wind areas are connected together, total generation will never come close to matching combined nameplate capacity. Generation will peak at between 60 percent and 75 percent of nameplate capacity. At this point the real cost per peak MW is higher than the nominal cost per MW. But since kWh generated is the same, real capacity utilization is also higher than nominal capacity utilization. So the cost per kWh does not change, but the quality of the power produced is higher. Higher quality power can make better use of transmission capacity.
Power quality improves even more when solar is placed into the mix.
This has implications for high renewable scenarios, 80 percent to 99 percent renewable. Scaling up transmission for such scenarios won’t increase transmission costs per peak MW, and might reduce them. Even if multiplying renewable generation by 3.3X increases additional transmission by that amount compared to a 30 percent scenario, it won’t increase line miles by 3.3X. Much of whatever additional capacity is needed will be provided by shipping more power along the same route required in the 30 percent scenario. And increasing the capacity of a line is a lot less expensive than increasing the mileage. For one thing there is no additional land required, and even the line itself does not triple in cost when it triples in capacity.
Additionally, tripling generation won’t quite triple transmission requirements. Much additional generation is a substitute for more expensive storage. But if we ever make really massive use of renewables we will need significant storage. Storage will increase energy quality, and thus reduce at least slightly the need for additional transmission.