Designing net-zero energy systems

This blog introduces some net-zero emission, renewable energy system design studies by Mark Barrett, Tiziano Gallo Cassarino and Ed Sharp. Most of the work was undertaken as part of the CREDS Heat theme, led by Bob Lowe.

Green Light (GL)

In the Green Light report (GL), Mark Barrett develops eleven 2050 net-zero system designs with variations of climate change, central and low demands, efficiency, heating shares and nuclear generation. GL uses a model (ETSimpleMo) of the UK, without interconnector trading. The system capital, operational and fuel costs are calculated and the designs optimised to find least cost. The transition of capacities, operation and costs between 2020 and 2050 is modelled.

GL explores how energy service demands might evolve with climate change, particularly reduced heating and increased cooling in buildings and vehicles, and how these demands can be electrified. Much electrification is direct, using heat pumps and electric vehicles and other equipment, but some is via electrofuels including hydrogen for industry and ammonia for ships. Reversible heat pumps are selected as a resilient adaptation option to provide heat and cool. District heat with heat storage is deployed.

Aviation is the toughest sector as it requires a hydrocarbon fuel and causes high altitude warming. In GL, some biowaste is available for fuel, but most is fossil kerosene. Aviation is the main residual emitter, and it requires balancing negative emissions, provided here with 20 GWe of direct air capture with carbon sequestration (DACCS) running on surplus renewables. Even assuming that aviation historic growth is halved, aviation fuel and emission balancing constitute about 20% of the total energy system costs.  A preliminary technical and economic assessment of the alternative synthesising renewable kerosene from atmospheric CO2 and electrolytic hydrogen is made.

The GL electricity demand is around 800 TWh and is mainly met with offshore wind (80%), onshore wind (4%) and solar (10%). Nuclear is assessed as economically uncompetitive so it is restricted to Hinkley Point C, the only plant now under construction. No biomass apart from waste is used, and this is allocated to aviation and some residual generation.

Across the hours of the year, demands vary with social use patterns and meteorology, and meteorology also drives renewable generation: sometimes the available supply will be greater than demand, sometimes less. An algorithm operates the system of renewables and stores hour by hour to meet demands.

Electricity, chemical or heat storage is needed to supply services when renewables cannot meet demand. The GL optimal storage mix comprises just 60 GWh of electricity grid input/output storage (e.g. batteries), and other stores which are not used for electricity generation – 1.3 TWh of electric vehicle batteries, 1.1 TWh of district heat storage, and 4.3 TWh of hydrogen storage for supplying baseload hydrogen to industry. These stores total about 7 TWh. An additional store of about 10 TWh of natural gas/biomass is used in 60 GW of peaking plant (without carbon capture) generating 6 TWh at a load factor of about 1%: this, which provides back-up of last resort, was found to be cheaper than generation fuelled with hydrogen or gas with CCS , even with the cost of DACCS balancing any small associated CO₂ emissions. The total GL storage is about 17 TWh.

A somewhat surprising result is that in the optimum systems, 20-30% of renewable electricity is spilled (unused) as it is cheaper to do this than build more capacity of storage or devices such as electrolysers and DACCS.

Using the same cost model, the total cost of Green Light’s net-zero systems is about the same as the current UK energy system. In GL, 90% of costs are fixed capital and operational costs, with the remaining main fuel cost being for aviation. Consumer heat pumps, the electricity network and offshore wind together account for 65% of the total system capital cost.

Thus consumers, apart from those who fly, would be almost invulnerable to fuel price spikes and import vulnerability caused by war or disasters such as Fukushima. However, like all capital invested in energy, vulnerability to changes in exchange and interest rates, and supply chain constraints will remain.

Long Term Storage (LTS)

The Long Term Storage (LTS) report (Barrett, Gallo Cassarino, Sharp) was written at the request of the Royal Society (RS) for its report Large-scale electricity storage (The Royal Society, 2023). LTS used hourly historic meteorology data from 1980 to 2010 to simulate, hourly, a system of electricity demands and heat met with heat pumps, and solar and wind generation. Total electricity demand averages about 700 TWh. 2010 was found to be the stress year because of low temperature and low renewables. About 65 TWh of 100% efficient electricity storage was required assuming all potential renewable generation were used. If renewable capacity is increased and some generation is spilled (unused) then much less storage is needed.

Royal Society (RS) report

Some of the information in LTS was used in the Royal Society (RS) report (The Royal Society, 2023). The RS report concludes that ‘tens of TWhs of very long-duration storage will be needed.’ and that ‘With the report’s central assumptions, this would require a hydrogen storage capacity ranging from around 60 to 100 TWh.’; this hydrogen would be made with electrolysis and used for generation.

The GL storage mix is very different from RS in that the grid electricity output storage is small, and the total GL storage is much smaller despite the GL demand being 800 TWh compared to 570 TWh of RS. Reasons for the differences include the renewable spillage and flexible DACCS demand in GL.

Unlike GL, neither the LTS nor the RS systems included certain important components, notably district heat with storage, negative emissions to balance remaining emissions from aviation, and interconnector trading, which would reduce spillage or storage need, or both.

Zero emission heating with renewables and interconnectors using ESTIMO

None of the above studies included international electricity trading. This aspect was addressed though CREDS research by Gallo Cassarino, Barrett and Sharp (Gallo Cassarino, Sharp et al., 2018; Gallo Cassarino and Barrett, 2021) using a model called ESTIMO (energy space time integrated model and optimiser), which simulates hourly the energy systems of the UK and four European regions and the electricity trade between them, and calculates costs. ESTIMO modelling showed trading could reduce storage needs significantly, by up to 30%. There were insufficient resources for ESTIMO to integrate negative emission in detail or apply optimisation, which plainly would be very challenging to do for the whole of Europe.

Discussion

A primary aim of this work is to show that renewable systems can be designed to reliably meet demands hour by hour in all meteorological conditions. A further aim is to find optimal least cost designs and estimate the required profiles of capacity expansion and investment to meet net-zero by 2050. The GL and ESTIMO studies find hydrogen heating not to be cost effective as compared to consumer heat pumps and district heating. Reversible consumer heat pumps were found to be a resilient heating and cooling option.

The work pointed to topics that might build on this or related research:

• The hardest problem is controlling aviation global warming and further analysis of the integration of renewable aviation fuel production into the energy system is needed.
• ‘Fast’ measures such as car downsizing, speed reduction and personal comfort systems rapidly reduce emissions and therefore aggregate emissions to 2050 – these need exploration given the climate emergency.
• It is complex to design strategies or algorithms for dynamically controlling renewable systems optimally hour by hour so as to minimise costs, and then to implement such strategies in real social markets.
• Environmental impacts such as land use, waste, and materials requirements are not detailed, and these impacts could significantly affect designs.
• Policies need to be formulated to deliver the capacity implementation rates to reach net-zero by 2050, taking into account supply chain capacities, consumer choice and so on.

Thanks go to the UKRI and CREDS for funding this work.

References

• Gallo Cassarino, T. and Barrett, M. 2021. Meeting UK heat demands in zero emission renewable energy systems using storage and interconnectors. Applied Energy, 306 (B): 118051. doi: 10.1016/j.apenergy.2021.118051Opens in a new tab
• Gallo Cassarino, T., Sharp, E. and Barrett, M. 2018. The impact of social and weather drivers on the historical electricity demand in Europe. Applied Energy, 229: 176–185. doi: 10.1016/j.apenergy.2018.07.108Opens in a new tab
• The Royal Society, 2023. Large-scale electricity storage, pdfOpens in a new tab

Banner photo credit: Guillaume Bourdages on Unsplash