To overcome intermittency and to transport renewable energy over long distances, energy has to be stored. There are many storage technologies under consideration. Examples include mechanical (e.g. pumped hydro, flywheel), electrical (e.g. battery, supercapacitors) and chemical processes. Pumped storage is very site specific and is localised. Hydrogen based and battery technologies appear to be the best options to compliment solar and wind technologies for stable and reliable energy supply due to the intermittent nature of renewable energy. Both these technology chains need to progress together for short and long term utilisation of renewable energy, however there are issues with each technology chain.
Battery technology is ideal for local storage and utilisation of renewable energy where long distance energy transportation is not an issue. Examples are residential dwellings/communities and commercial buildings and transport vehicles (cars, trucks, etc.). Typically energy use per day in a house hold varies from 2kWh to 20kWh and in poor countries it may be less than 1kWh. An electric car would require 25-100kWh storage depending on the distance to travel on a single charge (e.g. 100km to 400km). Thus battery storage an easily cater for this. Similarly battery storage in the MWh range can cater for local communities, commercial dwellings, etc.
One of the major advantage of battery technology is the efficiency losses for energy in and out are relatively small (typically less than 15%) thus increasing the overall efficiency to over 85%. Battery technology is reasonably advanced although increase in energy storage density, cycle life and overall life time would be beneficial in the long run. The major drawback is that for every kWh of energy storage, any additional kWh storage would require doubling the size of the battery and consequent weight.
Transmission of renewable electricity over up to few hundred km utilising existing infrastructure is another good option especially if it is complimented by battery storage to overcome intermittency of renewable energy technology either at generation or end use sites.
Battery technology, nevertheless has limitations for the transportation of renewable energy from generation (which may often be remote) to further away utilisation sites, especially over long distances with non-existent transmission infrastructure and overseas locations (up to several thousand km) where renewable energy availability is scarce.
Hydrogen offers a very good alternative to renewable energy storage, utilisation and transportation to almost every corner of the world. Hydrogen can be easily coupled to renewable energy technologies and its utilisation generates no greenhouse gases or pollutants. There is no limit to distance over which hydrogen can be transported. However, there are substantial challenges for its generation, storage and transportation including increased cost and efficiency losses at each step. The energy content of hydrogen per unit weight is very high but extremely low per unit volume unless it is compressed at very high pressures, e.g. to around 700bar or converted to liquid form (-253oC).
Over 60 million tons of hydrogen is consumed per annum globally and is mostly produced from natural gas. It is worth mentioning that this entire current hydrogen production capacity accounts for less than 3% of the total global energy demand. Thus a significant increase in the hydrogen production capacity would be required (and that too from renewable energy sources) to make an impact as a clean alternative to fossil fuels.
So far the major use of hydrogen has been for ammonia production, and in oil refineries and methanol production with very small use (less than 2%) as an energy carrier.
A large percentage of ammonia, produced globally, is currently used in fertiliser production (about 80%) with other uses including explosives, pharmaceuticals, refrigeration, cleaning products and some industrial processes. It has also been used in small quantities as a fuel for transport vehicles and for space heating. Ammonia is an excellent energy storage media with 17.5wt% hydrogen content and it stays in liquid form above about 9-10 bar pressure at ambient temperature. The infrastructure for its transportation and distribution is already in place in many countries and between countries. However, if ammonia is to be used as a renewable energy transportation media, technologies for its efficient production, its reutilisation as a fuel (direct combustion or in fuel cells) or reconversion to hydrogen need to be developed with minimal overall life cycle energy losses.
Clean hydrogen generation using water electrolysis with coupling to renewable energy is well established now, however, the issue is cost and economy especially when the cost of entire infrastructure is considered (includes cost of renewable energy input technologies such as solar panels or wind turbines, electrolyser, clean water supply system, maintenance costs and lifetime of each technology). About 45-60kWh electric input is required to generate 1kg of hydrogen. Thus the cost of renewable energy has to be very low per kWh. At best 80-90% efficiency can be achieved for hydrogen generation via electrolysis. Electrolysers varying in size from a few kW to 100s of MW depending upon the application requirement can be installed.
The next major step is getting hydrogen fuel for effective utilisation and transportation over distances and to countries and utilisation sites with low renewable energy footprint. This is a major challenge. For residential and small scale stationary power generation, it is not such a major issue as space is less of an issue and hydrogen can be stored at low pressures (10-30bar). For hydrogen utilisation in transport vehicles, many hydrogen / fuel cell vehicle manufacturers have opted for high pressure (350 to 700bar) storage in light weight storage tanks but efficiency losses for compression are significant in addition to cost of hydrogen compressors. The alternative of solid state hydrogen storage technologies where over 6-8 wt% storage is required including hydrogen insertion and extraction have not materialised so far.
For long distance and overseas transport, hydrogen transport in compressed form is not feasible. Thus its transport in liquid form (-253oC) or converting it to a chemical form such as ammonia and others chemicals is being explored. However, there are major challenges in terms of cost and also efficiency losses.
Hydrogen or its chemical form can be combusted to generate energy at utilisation sites or converted to electricity and heat in fuel cells. Again the typical electric efficiency of hydrogen utilisation in a fuel cell is 40-50% with 30-40% available as a heat component if required. Apart from the current high cost of fuel cells and some lifetime issues, the lack of cost effective hydrogen storage technologies and distribution infrastructure is hampering the mass penetration of hydrogen as an alternative transport fuel.
For each area discussed above there are a number of sub-set technologies which all have a number of challenges.