Rise of solar - balancing technology solutions
In April 2014 the U.S renewables industry made international headlines when the Energy Information Administration's (EIA) monthly figures revealed that the country's solar capacity had increased an unprecedented 418% from 2010 to 2014. Marking the watershed moment, commentators at the EIA stated that 'U.S. solar capacity has moved quickly from a relatively small contributor to the nation's total electric capacity into a one of comparative significance'.
Although today solar power only provides around 1% of the U.S' total energy needs, reported planned solar capacity additions indicate continued growth. Indeed solar will play a key role in helping states with aggressive renewable energy mandates such as California and Colorado meet their legal obligations. However with the advent of much intermittent solar capacity, the balancing task of fuel-based generators is rapidly getting more challenging. When the sun sets, the output from photo-voltaic cells (PV) goes to zero while electricity demand generally increases. But what technologies are capable of the balancing task and can be installed fast enough to keep apace with the rapidly expanding solar industry? Which solutions are affordable and support maximum renewables integration?
At face value, hydropower, which already provides around 16% of global electric energy demand, is an excellent choice to fill the gaps in renewables production. Such facilities are capable of starting and reaching full output in about five minutes, fast enough to react to a rapid decline in wind and solar output. However hydropower plants are concentrated in areas such as Finland and Norway with high precipitation and substantial differences in elevation. Other areas, including large portions of the U.S and western Europe have already maximized their hydropower potential.
Another much-lauded solution is energy storage, as it allows electricity produced with solar panels during the day to be used in the evening and abundant energy produced in the summer to be stored in the winter. However, this method's immaturity and prohibitive costs mean it remains a work in progress rather than an immediate solution. Technologies including batteries, flywheels, compressed air, pumped hydro and hydrogen may in time prove suitable for short term energy storage for a few days in locations with a guaranteed sunshine every day or with winds without long doldrums. But the caveat of these storage options is the cost. Batteries, for instance, would increase electricity prices for about $0.40 per kWh. Moreover, there are prolonged time spans where renewable energy sources have limited output, in latitudes over 45° for instance, where sunshine is severely limited in the winter season. Here storing energy for several months would be necessary and this is simply out of reach.
Demand-side response will play a role is managing short-term balancing, but finding switchable load capacity is near impossible. To create a demand response of 1 GW with smart appliances in households, at least 500,000 active washing machines or laundry dryers of 2KW each have to be controlled. With an estimated utilisation factor of 5%, the reality is that 10,000,000 of such smart appliances are needed for creating the 1GW demand response.
Given the limitations of the balancing techniques above, it would be wishful thinking to assume transmission system network operators could balance decarbonised electricity grids in the absence of fossil fuels - particularly in cases such as Hawaii where renewables are expected to make up 40% of the total electricity generation portfolio by 2030.
It is widely agreed that the thermal power fleet needs to be more flexible to accommodate renewables, and there is also consensus that natural gas offers a cheap form of peaking power. Currently jurisdictions such as California and the UK are planning to install the most efficient combined cycle gas turbines (CCGT) and open cycle gas turbines (OCGT) in order to ensure electricity system reliability. However such systems must ramp up over a number of hours and then part-load their output in order to support intermittent renewables production. This practice is both inefficient and expensive, doubling power plant operators' costs per kWh if their facility is run at 50% load.
Fortunately, another viable and readily available gas technology can be utilized in new generation infrastructure that offers a cheaper, more efficient and eco-friendly solution. Pro-renewables jurisdictions such as Texas are waking up to the benefits of fast-reacting gas-fired power plants based on combustion engines as the most plausible solution for renewables integration. These engines have lower temperatures that enable them to start up in one minute and reach full load is less than five minutes. Due to fast reaction time, these power plants are able to follow the output of renewables very closely. This is key to back up renewable energy, and, indeed, to enable more of it.
Moreover, the investment costs in combustion engine systems are relatively low due of uniformity and series production. Lead-time for engineering, procurement and construction is also short - as little as 24 months - because of standardized units, meaning electricity can be sent to the grid in a timescale that keeps apace with the fast roll out of renewables. Upon completion, the cost of running combustion engine plants in areas with a high penetration of renewables is also remarkably cheaper. In the UK, where there is a 30% renewables generation target specified for 2030, these engines could save the Department of Energy and Climate Change (DECC) up to £1.5bn, in comparison to CCGT.
Combustion engine technology is essential also in reducing carbon emissions. In a simulation for California, a recent study by Wärtsilä and Energy Exemplar showed that replacing 5.6 GW of planned gas turbines with combustion engine power plants cuts CO2 emissions 1.1% by 2022. This equals to about 500,000 short tons of CO2 or taking 100,000 cars off the road.
This is minuscule compared to indirect carbon savings by enabling more renewable energy. To install significant amounts of intermittent capacity, flexible backup is imperative. With current technology, fast-starting engine-based power plants seem to be the only realistic alternative. In other words, agile power generation may be the only way to reach CO2 reductions of 80% and more.
While the U.S is officially targeting 16% as part of its overall renewables generation by 2040, the country's recent Renewable Electricity Futures Study showed that renewables could potentially play a far more significant role in electricity generation, with projections of up to 80% clean power supply deemed to be feasible from technologies commercially available today. With such great potential hastened by policy both at the federal and state levels to reduce greenhouse gas emissions, there is an urgent need and opportunity to address the inherent volatility synonymous with renewables and to swiftly introduce balancing solutions that enable installed wind and solar capacity to be fully leveraged.
Flexibility to achieve system balancing can come from a variety of sources, though these challenges cannot be solved through hydro-power, energy storage and demand side response with full abstinence from fossil fuels. Nevertheless, when utilised using combustion engine technology natural gas is a cost effective, low emission and reliable renewable energy enabler - enabling the provision of dispatchable peaking power irrespective of geographical location, local climate and load capacity requirements. As the U.S transitions to a low carbon energy system, combustion engines can also free up traditional thermal plants to operate in their most efficient baseload capacity, creating further emissions savings that align with the increasing realisation that the core fundamentals of renewable energy generation can be exploited sooner rather than later.