Solar energy is such a vast energy resource that it can be used for any of our everyday needs, including electrical power, heating and cooling, water heating, industrial process heat, cooking, transportation, fuel production and even environmental clean-up. It comes to us as radiation which is pure energy (no mass associated with it), which is the highest form of energy and can be converted to many other forms for our everyday use. A tiny fraction of the solar energy that falls on the earth is sufficient to take care of all of the needs on the earth. However, it is available only during day time, and even during day time it is not available during cloudy and rainy periods. So it is obvious that if we are to depend on solar energy for our needs, we must be able to store it efficiently and cost effectively. By the way, wind, biomass, ocean energy, hydro etc. are the indirect forms of solar energy, that is, nature converts solar energy to these forms for our benefit. Even fossil fuels are a stored form of solar energy, which nature stored for us over millions of years for use during emergencies (that is our savings account), although we as humans have been using them exclusively and exhausting them, while throwing away the incoming solar energy (our income).
So the research goals for solar energy have been and continue to be:
1. Increase the efficiency of conversion to other forms while also reducing the cost.
2. Store energy more efficiently and reduce the costs.
3. Find newer ways to convert solar energy to useful forms.
Since electricity has become the preferred form of energy for almost all of our needs, a major emphasis in research has been on increasing the efficiency of conversion to electricity and reducing the costs. There are two main methods of converting solar energy to electricity – photovoltaics (PV) and solar thermal power (commonly known as CSP, acronym for concentrating solar power).
The present trend in research in PV is focused on using earth abundant materials for PV, since some of the materials in today’s PV panels, such as, Cadmium, Tellurium, Gallium, indium, selenium etc. are not abundant and also become hazardous waste at the end of panel life. Silicon, which is still the major material used in solar cells is, of course, available abundantly on earth. Solar cells using earth abundant materials that are being researched include, Dye Sensitized Solar Cells (DSSC), polymer solar cells and Perovskite solar cells (PSC). The areas of research in these cells includes improving their efficiency and stability over time.
Since costs of PV in the last 10 years have come down so drastically, CSP has not been able to compete commercially in the world. However, we are confident that CSP will become competitive eventually. CSP has two big advantages over PV:
1. It uses the same thermal power conversion as the conventional thermal power (fossil fuel or nuclear based) and can therefore be integrated with the existing power infra-structure easily.
2. It uses thermal energy storage which is about one tenth the cost of battery storage.
In order for CSP to become cost competitive, the conversion efficiencies must increase and the cost must decrease. Since thermal power conversion efficiencies increase with an increase in the temperature (second law of thermodynamics), the trend in research is to improve the central receiver tower (power tower) technology to increase the conversion temperatures to around 700 C-800C. And since it is not practical to use steam at such high temperatures, present research is to find a replacement for steam. Supercritical CO2 (sCO2) is a very attractive fluid for this replacement. Simulation has shown that operating a sCO2 Brayton Power Cycle at about 750 C can give a conversion efficiency of more than 50%, something you could expect only in a combined cycle plant operating at more than 1100 C before. So most of the present research is directed toward making sCO2 power conversion technology practical.
For completeness sake, there is a lot of research going on in other solar energy applications also, such as, solar desalination, solar refrigeration and cooling, solar photocatalytic environmental clean-up, daylighting and passive solar uses.
An essential part of increased and complete use of solar energy is storage during the time sunlight is available, so that it can be used during the times sunlight is not available. There are many ways of storing solar energy, including batteries, thermal energy storage, pumped hydro, compressed air etc. The main research trend is to reduce the cost of battery storage and thermal energy storage (TES). Thermal energy storage is by far the cheapest way to store energy ($10-$40/kWhth for TES vs $200-$500/kWhe for batteries). However, if you want to store electricity as heat, you need to convert it to heat first and then convert it back to electricity. Converting heat back to electricity requires additional equipment and has an efficiency of around 30%-40%, so $15/kWhth for TES would be equivalent to about $45/kWhe, which is still much cheaper than battery storage. The trend in electrochemical energy storage (battery is electrochemical storage) at the present time is in improving Supercapacitor storage. Supercapacitors can give you very fast discharge but have very low storage capacity. The present research is to increase the Supercapacitor energy density with the goal of bringing it close to the battery energy density. Another way to store solar energy is to use it to produce hydrogen from water and use hydrogen to produce electricity in a Fuel Cell when you need it. Research is also going on this area.