Science for Renewables

I was recently asked to write how I see the contribution of science to the development of renewable energies in the coming 50 years.

Science for 100 % renewable energy in Europe before 2050

Dr. Frank Dimroth, Fraunhofer Institute for Solar Energy Systems ISE, Freiburg, Germany

In view of the severe potential impact of climate change on the living conditions for our children and grant-children, it is our immediate responsibility to act and perform every necessary step to transform our energy sector into a sustainable and carbon free one. This implies that all carbon based fuel consumption must be stopped in the very next centuries. The urge of this transition implies that only accessible technologies can be considered. Waiting for a higher level of technology readiness or for scientific breakthroughs will increase the risk that the earth’s atmospheric temperature exceeds a critical limit, resulting in disastrous consequences for our foot production, infrastructure, human health, safety and economy. Therefore, all forms of accessible renewable energy technology including wind, photovoltaics, solar thermal, geothermal, hydropower and biomass have to be explored according to their potential to contribute to a sustainable energy mix. They need to be combined with new distribution and storage solutions, supported by appropriate market instruments to guarantee steady and continuous market growth. And we must stop subsidies for gas, oil and coal immediately.

For Europe the transition offers a huge opportunity. Currently, Europeans are spending more than one billion € per day for fossil fuel imports. The transition to renewable energies allows us, if political frameworks are set correctly, to revert our energy supply to local value creation and local resources. Solar energy systems or wind turbines can be produced everywhere in Europe, creating employment and replacing fossil fuel imports. What a chance for all those regions which are suffering from high unemployment. Also security of our energy supply and overall safety are significantly improved by terminating dependency on fossil fuels. We should be confident in our ability to manage this transition successfully and translate our energy system for the benefit of our society. We exported the model of industrialization based on coal and oil. It is time for us Europeans to take the lead and provide a new model of a modern civilization based on renewable resources.

Having said that the transition of our energy system to zero carbon emissions is possible with today’s technologies, this also means that no scientist is needed to achieve this goal. But of course the transition should be obtained at the least amount to disruption, technological failure, lowest cost and with the most intelligent mix of technologies. We still need to define this optimum path and this has to include technologies which cover sectors from electricity production to mobility, heat generation, air conditioning, energy storage, water treatment as well as fabrication of fertilizers, polymers or other products based on fossil fuels. This will be a continuous task of adjustments based on the current state of knowledge. Research and development provide the necessary scientific basis for defining the optimum path and will have three important functions. We as scientists have to identify a wide range of technology options with their specific benefits and risks. We need to understand how these technologies can best work together and how they can gain wide acceptance. This needs a highly interdisciplinary approach. Second we have to engineer existing technologies to reach higher efficiency, lower resource consumption and lower manufacturing cost. This requires focused and applied research directed towards well defined targets. And third we need to identify and invent new technology options to facilitate the transition process. This will require basic and interdisciplinary research with a minimum of constraints. In conclusion, scientists are by no means useless in this process but create the necessary knowledge and innovation to minimize failure and cost. Having in mind the tremendous size of the problem, Europe should use every accessible opportunity to increase knowledge and understanding on this way. This includes putting clear priorities on research in the energy sector and recruiting the most talented young scientists to work on solutions for a sustainable energy economy.

Within the EASAC study the question was raised to identify potential breakthroughs in sustainable energy technology which are expected until 2050 and towards the end of the century. I concentrate on discussing the first period which will be the most important one in shaping our future energy system. Based on my working experience, I am most familiar with the field of photovoltaics (PV). PV offers direct conversion of sunlight into electricity by using typically semiconductors. Recent years have brought an incredible reduction in prices which will continue to decline. PV electricity today can be generated for 10-15 cent/kWh depending on the location ranging from South of Spain to North of Germany. Price reduction was accelerated by international market pressure, overcapacities and improvements in technology. Silicon has increased its market share compared to thin film and other technologies and provides close to 90 % of all solar cell products today. New technologies are facing difficult cost challenges when entering the market, starting at low volume and typically higher cost. But, conversion efficiencies for single-junction Si solar cells are currently fixed below 25 % (without concentration) and have not improved over the last 13 years. The coming decades will show that manufacturers and scientists are seeking new technologies to overcome this boundary and push the efficiency towards 30 %. This is naturally possible in multi-junction devices which separate the broad sun spectrum by using different bandgap absorbers. It is not an easy task to combine such approaches with silicon but III-V crystal growth on Si has been making considerable progress in recent years. Also nanomaterials like quantum dots in SiOx or SiC are investigated to form high bandgap absorbers on Si. Combinations of amorphous and microcrystalline Si as well as Si and II-VI or Si and polymers are further possible solutions to form tandem devices. I believe that tandem solar cells on Si will reach efficiencies of 30 % in 10-15 years from now. This will allow harvesting significantly more power from the same module area. Additionally, silicon PV modules will be developed to last for 30 years or more which will lower electricity generation costs considerably.

The highest photovoltaic efficiencies of 44 % are achieved under 500-fold concentrated sunlight with tandem solar cells made of III-V compound semiconductors. Record efficiencies will further increase to approx. 53 % in 10-15 years making III-V multi-junctions the highest efficient PV technology. The main application for these devices is in concentrator photovoltaic systems (CPV) which will penetrate the sunbelts of the earth with high direct normal irradiance above 2000 W/m2/year. Here they compete with solar thermal power plants (CSP) but offer the advantage of higher efficiency and no cooling water demand. These may be important factors which favor the technology compared to CSP. I believe that solar power plants in the GW range will cover the sunbelt regions of the earth in 20-30 years from now providing low cost electric power to be transmitted to the industrial centers and for converting solar energy into renewable fuels. Research will provide the necessary improvements in solar cell efficiency to lower the cost of CPV systems and to improve overall system components and reliability. Concentrator photovoltaics has the potential to provide electricity in the 5-6 cent/kWh range if used in regions with direct normal irradiance of 2500 kWh/m2/year.

Reducing manufacturing cost and replacing scarce materials with more widely available alternatives are key targets of photovoltaic research and development. This includes replacing e.g. Ag metal by copper or aluminum. In silicon, the use of metallurgical grade feed-stock is an option to reduce energy intensive fabrication processes. Si solar cells with thickness in the 20-40 mm range may be transferred to new functional substrates which offer low manufacturing cost and functionalities like photon scattering or up-conversion. Breakthroughs in technology and new material combinations may allow Si thin film solar cells to reach both, higher efficiencies and lower manufacturing costs. Si or III-V solar cells are also developed in nanowires which reduce material consumption and allow integration of III-V on Si without the constraints of lattice matching. This may offer an alternative approach for low cost multi-junction absorbers on Si if the growth processes can be applied to large areas. New materials will be integrated into advanced photovoltaic cells and modules to improve their performance and lifetime. Encapsulants, broad band anti-reflective coatings, transparent conductive oxides, improved semiconductor compounds, more stable polymers are just some examples where photovoltaic research will benefit from advances in material science. Material science lies at the heart of innovation in photovoltaic research and development.

II-VI thin film solar cells from earth abundant compounds like Cu2ZnSnS4 will be developed to reach efficiencies in the 20 % range and offer opportunities to be deposited on materials which form e.g. facades or roof tiles. The same may be true for polymers if material stability is improved. With the large amount of material options in the field of organics, there is still significant room for innovations and it is expected that multi-junction absorbers with improved reliability may open a new market of photovoltaic elements which cover large area applications. Building integration of photovoltaics on roofs and walls will become the standard for all new houses in 10-15 years from now. This includes higher flexibility in shape and design of PV products which will be more pleasing to the eye and therefore attract larger markets.

Photovoltaic applications will become the standard in residential houses throughout Europe. Higher PV module efficiencies and new applications in facades will increase the electricity generation of typical family houses to 5-8 MWh/year). This is sufficient to supply energy to all consumers including heating and cooling if houses are well isolated. DC electricity may be used to avoid inverters and improve overall efficiency. Electric heating and cooling is supported by heat pumps which produce 6 kWh of heat from 1 kWh of electricity (e.g. with geothermal probe or solar thermal collector). Such technologies are already available today. Local energy storage has been demonstrated in batteries, electrolysis of hydrogen or mimics photosynthesis of liquids like isobutanol. Research will lead to innovation and new solutions to reduce cost and improve reliability of such systems. Existing technologies must be further developed to use non-toxic and earth abundant materials before starting widespread application. The field of electricity storage is certainly one in which scientific breakthroughs are most needed in the coming decades and the success of local solutions will depend on these future developments.

Energy which is stored locally can be used for multiple purposes including mobility, conversion into heat or conversion back into electricity. This will be the basis for an independent energy supply of single houses or neighborhoods to match the irregular production of wind and solar with the actual demand cycle. In conclusion, decentralized production and consumption of energy will be one of the most important achievements on the way to a renewable energy system in Europe. Single houses or neighborhoods will become independent energy factories, able to produce, store and deliver electricity depending on current prices and supply. Intelligent software will optimize local electricity production, consumption and storage as well as trading of available resources with the overall network. An easy to use and automatic energy trading system should be established to allow all EU citizens to invest and benefit from such developments. This will foster investments into new technologies, stabilize fluctuations in the electricity production by variable market prices and allow every European to benefit from the transition to renewable energies.    Dr. Frank Dimroth / 3.3.2013