High-density energy through the use of fossil resources and the consequential threat of greenhouse gas (GHG) emissions to global warming have paved the route for innovative ways to harness energy from nature, particularly the sea, wind and the sun.
This has provided some hope that high polluting thermal generating sources using fossil fuels, can be replaced with cleaner renewable sources. However, there are still issues of how we can efficiently harness them due to variability in supply. Thus, the challenge shifts to ensuring a viable economic argument and a stable supply of electricity from the system assets that have been boosted by government subsidies.
One metric that is used to assess this is the Levelised Cost of Electricity (LCOE) for the entire lifecycle of the technology. The other is the Energy Return on Energy Invest (EROI), which is the ratio of energy generated and delivered by that source, to the energy that is used from that source. Using such metrics helps to understand and compare the life cycle cost and economic sustainability of each generation source.
When assessing the technical challenges including the variation of renewable energy generating sources, in comparison to traditional power plants, the key points to consider are:
- Variation/fluctuation in the availability and forecast predictions.
- Modular nature of construction.
- Location dependency is due to choosing the best location to harness the energy.
- Conversion/inverters are required to counter the non-synchronicity of the power generated and to stabilise/make it compatible for connection to the grid.
- The distance required for transmission to the transformers.
The above points are reflected in the life cycle costs; specifically, they are apportioned into utilisation costs, balancing costs, connection cost or LCOE and grid costs.
Utilisation costs reflect the true nature of the system and the factors that result in higher or lower lifecycle costs. What is more of a concern are the variabilities of the primary sources and the conversion of each source to electricity. This directly affects the utilisation costs and how the energy mix has to mitigate for any load deficit i.e. complementing the system with storage mediums, smart grid forecasting or more conventional methods such as the use of gas turbines to boost supply. Such complements contribute to the systems’ costs. The greater the capacity contributed by renewable generating sources, the higher the utilisation costs. As such, the need to diversify renewable sources is paramount to a country’s generating mix. The proceeding section describes the various renewable energy sources.
Renewable Energy Sources That Can Be Incorporated Into an Energy Mix
Bioenergy, specifically biomass, is used to describe the energy from fuel derived directly from trees and plants. The fuel may be grown for power generation or it may be waste such as straw from cereal farming, bagasse, rice hulls, maize husks or wood waste.
The process is contentious in its definition as a renewable form of energy. This is based on the fact that the energy input into the process of growing, fertilising, harvesting the crops and processing them can require more energy than the output energy from power generation. Even more so are the emissions generated from burning the products, particularly wood pellets and the fact that GHG emissions are released quicker than they are absorbed.
The environmental impacts that relate to the sustainability of land use and management make classing them as renewables, very debatable. There are even questions relating to their use and the rise of global food prices. Nonetheless, the method continues to be part of energy policies in places like the EU.
Currently, bioenergy accounts for 13 to 14% of the total energy consumption. It is also the largest renewable energy source globally. As a whole, it contributes 2/3rd to the renewable energy mix. (World Bioenergy Association). In the UK, 1.6% of arable land is used to grow bioenergy crops. In 2019 alone, the UK generated 37,314-gigawatt-hours – equal to over 6.7 million tonnes of oil of plant biomass were used to produce electricity and heat in the UK. (Crops Grown For Bioenergy In The UK, 2019). Some of the UK’s biomass energy comes from existing coal fire plants, which have been converted to burn wood.
Solar energy comprises photovoltaic (PV) and low-temperature heat. PV presents the most common method for utilising the sun’s radiant energy and light. The energy and sunlight act on PV cells thereby producing electricity. In recent years, large-scale utility-based PV farms have been constructed in the hundreds of MW capacities all over the world, with global capacity reaching 627 GW at the end of 2019. The International Energy Agency (IEA) states that solar energy is set to break records for new global deployments every year after 2022. On average, a 125 GW increase in capacity is expected globally from 2021-2025. Currently, the LCOE of solar photovoltaics is 3.12 to 11.01 €Cent /kWh. This can vary depending on the type of plant and the solar irradiation. China continues to be in the lead, it has the world’s largest solar capacity – 253 GW were installed at the end of 2020. In 2021, it further increased its capacity by 49 GW. The United States of America added 15 GW by end of 2020 as well, its solar capacity has also surpassed the 100 GW mark.
The efficiency is greatly improved by installing rotary trackers in the modules. Integration issues include cloud cover, which can reduce output by as much as 60-80%. Another issue is module degradation of the cells by as much as 0.5%/year, with manufacturers providing guarantees for 20 to 25 years. The materials and manufacturing processes pose sustainability questions. The materials range from glass, copper and plastic to more exotic and toxic compounds such as cadmium telluride, copper indium gallium selenide, silicon tetrachloride and anti-freeze. The types of materials employed are dictated by the type of solar cells (silicon or thin-film). Disposal and recycling of solar PV cells will pose some environmental challenges, which have not been fully addressed.
Concentrated Solar Power
Concentrating solar power requires the sunlight and uses a tracker and mirrors/heliostats to follow the sunlight and reflect/concentrate it on absorbers. The available energy is used to heat a working fluid. The working fluid can be oil, water or molten salt, which can be incorporated into a secondary circuit of a Rankine cycle to complement a conventional steam cycle power station. Another use of CSP direct sunlight is for heating and cooling homes and buildings using a refrigeration cycle.
In 2019, a 600 MW increase in CSP capacity was observed – a 20% decrease in comparison to 2018. Israel led the growth (230 MW) with china second in lead (200MW) and South Africa third (100MW). Kuwait also added 50 MW of operational CSP year-end 2019 (IEA tracking Report, 2020).
Overall, CSP generation increased by 35% in 2019. While this growth is exceptional in comparison to the 24% mean average for 2011‑18, it is still not up to the mark with SDS. To reach the SDS target, annual capacity expansions need to reach 8GW each year by 2030 (IEA tracking Report, 2020).
IEA states that growth in CSP is forecasted to come from emerging economies like China, Morocco and South Africa. Even though China introduced an ambitious target of 5 GW by 2020, the deployment of these pilot projects has been slow at best.
Wind Energy capacity has increased considerably. The cumulative installed capacity of wind energy has reached approximately 743 GW in 2022. Moreover, onshore wind capacity was the highest last year, reaching over 700 GW. Last year, China broke the world record for the most wind power capacity installed in a single year – 52 GW of new capacity, double in amount to the country’s yearly installation compared to 2019.
In 2020, the global weighted-mean LCOE (Levelised Cost of Electricity) from new capacity additions of onshore wind declined by 13%, compared to 2019. Likewise, the LCOE of offshore wind fell by 9%.
Low utilisation and intermittency problems mean backup systems are required to provide the residual load to meet demand. Wind turbines are only able to harness between 70-80% of the available kinetic energy due to environmental factors imposed on the turbine design as a result of the conservation of mass and momentum of the fluid stream (Betz’s law). The construction comprises a tall tower usually made of steel to mount the nacelle that houses the generator, and is coupled to rotors with 3 aerofoil-shaped blades. The entire assembly is housed within reinforced concrete foundations. Turbines require wind speeds of about 9-56mph although this range is not achievable in most areas, it is enough to harness a capacity factor of 20-25%.
The UK’s current Wind capacity stands at 24.6GW. In the years 2008 to 2020, wind energy capacity in the UK has increased approximately by 3.5GW. Wind Power generation contributed 24% of the total electricity generation in 2020 for the UK. Offshore wind sources accounting for 13% while onshore wind sources for 11%
However, a backup system as previously mentioned is required to support wind turbine systems. Maintenance of wind turbines is an issue in terms of cost because this involves removal of the entire tower, generator, rotor and blades. Furthermore, the operational life is typically 30 years meaning that a vast amount of turbines in Europe will need to be replaced in the next decade. The unknown decommissioning costs present a big problem in terms of estimating the LCOE. This means that significant margins are added to the overall price/MWh.
Known as one of the oldest and most reliable renewable resources, electricity from hydropower (when flow conditions allow) can be an efficient renewable way of producing electricity. A considerable amount of mechanical power can be generated with stable inertia, which means that generation can be synchronous and not require backup like wind and solar. Global hydropower capacity currently stands at 1.3 TW in 2020 representing year-on-year growth of 1.6%. Even though this figure is higher than 2019, it is down by 2% to tackle and contribute towards lessening climate change. China has given immense priority to hydropower development, it reached a total hydropower capacity of 380 GW by 2020 and aims to increase this to 470 GW by 2025.
Hydropower is ideal for peak load backup for solar and wind. A big fraction of the world’s renewable electricity generation comes from hydropower. The primary advantage of hydropower is its ability to deliver high peak loads at any point in the season. This is because of the relatively short time it takes to start the turbines. However, this ability is dependent on the type of hydro system. High irrigation requirements will limit the flow that is diverted for power generation; in other instances, run-of-river hydropower plants may have reduced seasonal flow. At present, hydropower contributes 30 to 40% to UKs renewable generation. Globally, hydropower energy contributes 17%.
Geothermal energy is the heat that is sourced from the body of the earth. It is location-specific and is based on hot steam, which can be brought from deep below the ground and used to drive a steam turbine. Specifically, the earth’s crust is in the region of focus and is approximately 5 to 55 km (3-34 miles) thick, with the temperature increasing by an average of 17 to 30°C for each km below the surface. Geothermal energy has a relatively low running cost and is always available. Key locations for geothermal energy include Turkey, Iceland, New Zealand, Indonesia, Italy and the USA.
The worldwide geothermal power capacity in 2019 was up to 15.4 gigawatts (GW), of which 3.68 GW (23.86%) are installed in the United States.
Typically, the areas with potential for geothermal energy range between 150°C for hot dry rock geothermal to 400°C, at depths of 4-5 km. Figure 1 is a picture of a geothermal plant next to an area of hot springs in Iceland
Ocean energy can be covered in 3 different technologies, which are all designed to harness energy from seas and oceans. Specifically, these are ocean currents also known as the ocean or tidal stream, wave energy and Ocean Thermal Energy Conversion (OTEC). These forms of energy are all in early demonstration phases and have shown promises of potential full-scale commercial deployment. Ocean current or tidal streams require the tides to be harnessed with barrages in a bay or estuary.
Turbines are placed close to the barrages and are turned when trapped water is released in either direction. Whilst this has been achieved in Canada, France, Russia and South Korea, the technology is constrained environmentally, which restricts worldwide deployment and has also led to the Swansea Bay Tidal Lagoon project being shelved in 2018. Nonetheless, there are still other forms of tidal energy being explored such as tidal arrays. The technology as a renewable source of energy is constant hence the potential for predictability due to the currents generated. Estimation of global energy dissipation due to tidal is approximately 22,000 TWh with 200 TWh considered economically recoverable and approximately 0.6 TWh converted into electricity.
Wave energy technology harnesses the energy from the surface waves around coastal and nearshore sites. Portugal has the first commercial plant (Agucadoura Wave Farm), which benefits from fluid being pumped at high pressure through turbines. The farm is capable of producing over 2.3 MWe. In the UK, it is estimated that wave, combined with tidal has the potential to deliver approximately 20% of the UK’s current energy demand. However, the technological risks such as survivability of the equipment, the uncertainty in recoverable energy and wind power having more coverage at the expense of ocean technologies, have limited further R&D investments in projects such as the Pelamis sea snake and the Oyster hinge-flap.
OTEC (Ocean Thermal Energy Conversion) depends on the principle of using a heat source and cold source to drive an engine. The heat source is at the surface of a tropical or subtropical sea, whist the cold source is in the depths of the sea. With typical surface temperatures of 24°C to 33°C and temperatures of 5 to 9°C at depths of 500m, this provides an exploitable temperature difference of up to 28°C. However, losses in the process mean that it is likely to be approximately 20°C, allowing for a conversion efficiency of 3-4% when pumping of the cold water is considered. The typical depth of 500m would need to be increased if there is a risk of the cold fluid becoming warm. The challenges associated with the temperature variation and the effect of the variation on the geometry of the pipes particularly the cold water pipe, including choice of cycles means that only a handful of projects are currently in development.
Accounting for Green House Gas Emissions
Electricity generating systems contribute two-thirds of global GHG emissions. However, quantification of their total GHG emissions can be a challenge due to differences in adopted methodologies, assumptions, and input figures relating to processes. Furthermore, of the GHGs, only carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) are considered using a carbon equivalent (CO2eq) unit of measurement. The Life Cycle Assessment (LCA) method is most widely used. Figure 2 shows the accounting scope while Figure 3 illustrates the LCA method (Imran khan, 2019).
Table 1 shows research by the Intergovernmental Panel on Climate Change – the life cycle CO2eq of typical generating technologies (current commercial and pre-commercial technologies), in order of the highest median value.
Challenges for Renewable Energy in the UK
Concerning Table 1, the data suggests that out of the commercially deployed renewable technologies, wind contributes the least amount of GHG emissions (CO2eq/kWh), with biomass (dedicated) contributing the most amount of emissions.
Biomass with coal is not considered due to the combined fossil fuel usage. An observation from the figures indicates that they do not take into account the intermittencies in operation for technologies like wind power. Due to grid priority provided for wind and solar as a result of their low capacity factors (weather relating), backup load-following systems are required to provide peak power to maintain a steady output. Such backup sources are usually provided by baseload sources such as nuclear or hydropower, which are dependable.
The effect on other sources within the energy mix includes increases in CO2eq/kWh due to rapid increases and subsequent decreases in power. This effect is not accounted for in the system and emission costs because wind and solar costs are isolated from such effects.
Concerning bioenergy/biomass, it is labelled as a carbon-neutral form of energy generation based on the fact that it involves the burning of natural products. However, the GHGs are the same as products released from the burning of fossil fuels. Such products include sulphuric oxides and nitrous oxides as well as carbon monoxide and carbon dioxides. Forestry incineration is particularly controversial because the released carbon dioxide gases have been accumulated over long periods and are not necessarily reabsorbed quickly by trees and plants.
Significant variations between min and max CO2eq/kWh is an indication of the level of technological complexities. It is also evidence of insufficient learning from experience (LfE) due to geographical and environmental constraints, disjointed and inflexible regulations or technological maturity. This is particularly the case with hydropower, of which all of the aforementioned reasons could apply.
Predicting future energy mixes with a huge emphasis being placed on renewables is challenging. This is due to the electrification of vehicles and the potential electrification of heating pumps to replace gas used for heating homes and buildings. The electricity demand will continue to rise. However, wind and solar which are weather dependent, provide the majority of the renewable capacity in the UK. Regarding the UK, it does not have a variety of renewable generating sources with large enough capacities (i.e. hydropower). The UK has a hydropower potential of 850MW to 1550MW. However, environmental concerns have limited their progress to large scalable sources. Instead, the focus is on micro-hydro systems in local communities. Concerns raised with tidal streams technologies have also hampered the development of the technology.
Renewable energy sources have different non-aligning sets of environmental costs when compared to fossil generating sources or nuclear. In addition, they take up a lot of areas due to their low energy footprint. Further to this, is the noise pollution and poor landscape from technologies like wind. More capacity from wind and solar will inevitably be added to increase their contribution to the UK’s energy mix but renewable solutions with reduced intermittency should be explored or its capacity increased in the case of hydropower to improve grid resilience in meeting future power demand.
Areas for Future Work to Support Renewable Energy Development in the UK
Research in the following areas can help accelerate Renewable energy development in the UK:
- Methodologies that account for GHG needs to consider carbon intensity analysis that varies with time. This allows the fuel mix to be optimised to maintain minimum emissions from electricity generation. It would also help in planning future grid expansions by maintaining a low-carbon grid.
- An appropriate and environmentally-friendly method of disposal and recycling of solar panels at end of life requires careful thought in terms of the impact on the environment. A robust plan based on R&D needs to be defined to avoid toxic materials contaminating the environment.
- Research needs to be conducted to provide a full emissions picture and the effect of other GHG emissions on the environment in addition to CO2, CH4, and N2O. Furthermore, a true emissions cycle including a timeline that accurately estimates when CO2 is absorbed by trees and plants, needs to be developed for biomass to improve the understanding of the carbon intensity and to provide better estimates of the CO2eq/kWh.
- Development of the hydrogen economy needs to be accelerated including an understanding of the total CO2eq/kWh and the LCOE/LCOH (Levelised Cost of Heating equivalent).
- Development of full-scale storage mediums such as hydro-pump and battery storage systems needs to be accelerated including an understanding of the total CO2eq/kWh and the LCOE.
- Small scale geothermal projects need to be commissioned in areas that have been identified as having potential resources (British Geological Survey, 2019).
- Investigate how leading countries have successfully and sustainably harnessed hydropower to understand if the environmental risks in the UK can be mitigated by improved infrastructure designs.
- Ocean projects, particularly tidal stream projects, need to be reignited and further developed based on knowledge gathered from recent UK projects and other parts of the world.
- Transitioning to smart grids, which will modernise the transmission and distribution systems to maintain a reliable and secure electricity infrastructure, capable of meeting future demand.
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