Optimal Spanish clean energy grid would require 75 GWh of battery capacity

Maintaining thermal power plants solely to provide grid-strengthening “ancillary services” implies an increase in electricity supply costs and CO₂ emissions. Red Eléctrica has calculated the cost of ancillary services in Spain last year was €2.67 billion ($3.13 billion), 7.5% higher than in the previous year. That cost came on top of related CO₂ emissions and a reduction in the share of renewable energy in the mix.

University of Seville researchers have developed a methodology to compare the economic impact and supply costs of providing ancillary services in realistic high-renewable-energy-penetration scenarios in Spain, as well as the related CO₂ emissions.

The scientists, in the “The cost of ancillary services in high PV penetration scenarios: the case of Spain” article, published in Energy Conversion and Management: X this week, considered a power mix with a high percentage of solar energy generation and analyzed the difference between ancillary services being provided by conventional power plants and by renewables sites and storage.

Various scales of renewable energy and storage deployment were analyzed in the search for an optimal energy mix. The calculations included the annualized investment and fixed operating and maintenance costs of wind-, photovoltaic-, and combined-cycle thermal plants; gas turbines, and storage. The researchers also considered the cost of natural gas and CO₂ emission allowances for combined-cycle plants and gas turbines, and calculated CO₂ emissions for each model.

Scenarios

A system relying on conventional power plants for ancillary services would require 17 combined cycle plants, the researchers found, with nine of them limited to 1,132 hours of operation per year.

A “constrained” scenario would require nuclear power and nine gas turbines operating year-round plus 130 TWh of solar and wind power and 60 GWh of storage. Some 2.1% of the energy generated – 2.7 TWh – would be wasted and the system would cost €8.36 billion, emit 9 million tonnes of CO₂ and would draw 62% of its generation from renewables. The researchers noted that would include €390 million to maintain the gas turbines, would entail lower renewables penetration and would emit two-and-a-half times more CO₂ than in the lowest-cost scenario they modeled.

Closing half of Spain’s nuclear capacity would theoretically make room for more clean energy and reduce the curtailment of excess electricity – because combined cycle plants are more flexible than nuclear reactors – but at the cost of more thermal energy generation and emissions.

An “unconstrained” scenario would involve investment in flexible alternating-current (AC) transmission systems (FACTS) to provide the ancillary services to help maintain voltage levels and system inertia. That would remove the need for combined cycle gas turbines, making for higher penetration of renewables in the energy mix and reducing cost and emissions.

“The amount of FACTS required is not that high, only 1,080 MVAr [megavolt-ampere reactive] of capacity with 3 MWh of storage capacity,” the researchers wrote.

The optimal volume of energy storage capacity for each megawatt of renewable energy generation capacity has been calculated as 3.5 MWh, with an 875 kW inverter. That energy storage capacity requires incentives to make its return on investment viable.

Optimal system

The University of Seville researchers concluded the cheapest-to-establish Spanish energy mix would comprise 80 TWh of solar- and 70 TWh of wind-power alongside 45 GWh of storage capacity: figures modeled in the unconstrained scenario above. Grid flexibility would be maintained by replacing nuclear generation with combined cycle gas plants. Once established, however, the cost of supplying that energy mix would be higher than in the optimal model and would also have greater emissions: Shuttering nuclear plants equates to 12.7 million tons of CO₂.

The optimal energy system would comprise the same volume of solar and wind power generation capacity but with 75 GWh of storage. That extra storage would require 2% more to establish than in the previous model – at a cost of €7.96 billion – and would see 13% of the clean energy generated (4.56 TWh) curtailed but would produce only 3.71 million tons of emissions: 58% less than with the 45 GWh of storage scenario outlined above. Renewables would contribute 68% of the energy mix under this optimal scenario.

From pv magazine España.