Most energy devices merely convert one form of energy into another. Take a diesel generator for example; it converts chemical energy into rotational kinetic energy for the alternator. However, even after 100 years in perfecting its design, the energy conversion efficiency for an IC engine (used inside a diesel generator) is a mere 15%. With turbo chargers it can be boosted to 18%-20%. These numbers are ridiculously low but are never really considered noteworthy due to the availability of cheap liquid fuels.
Similarly, a startling fact regarding the “Jekyll and Hyde” nuclear energy has never gained the deserved attention. It is largely unknown that wasted nuclear fuel rods have 95% fissionable uranium still inside them. Reprocessing the fuel rods is a possibility, but is still a long way away from a feasible reality. However, due to high energy density and low price of both oil and nuclear energy, their wastage more often than not gets overlooked.
Renewable energy devices have witnessed on and off development in the last 50 years. By observing their current energy efficiency while discounting the energy density, one can gauge how far they have progressed in just half the time compared with most machines that are in use today. Note that this article does not propound their utility or price effectiveness, it only highlights their potential. A list of top 10 renewable energy devices is compiled based on their efficiency ratings.
- Mono Crystalline Silicon Photo-Voltaic (PV)
Crystalline PV panels
Among the PV panels available in the market, crystalline silicon is the most mature with over 80% of the market share. The theoretical peak efficiency of PV is known as the Shockley–Queisser limit and it proposes a maximum of 33.7% for a material with a band gap of 1.34 EV. Silicon is the choice of the PV industry as it is the second most abundant element on the earth’s crust. The maximum possible efficiency for silicon is 29% due to its band gap being 1.1 EV. The panel efficiency has consistently improved particularly in the last 8 years. The most efficient solar panels currently available in the market are 21.5% efficient.
This industry has been encouraged by Feed in Tariff policies, which pay the owner of a PV system at a lucrative rate for every kWh hour produced and even more if it is fed to the grid. As a result, the price of PV has dropped by as much as 80% in the last decade.
- 3-Bladed Large Wind Turbines
3 Bladed Wind Turbine
The large wind turbines due to their better automation and controls are able to harness 27%-30% of the energy in wind. It is due to their superior technology of gearing systems, in-wind positioning and adjustable blade angles that their performance supersedes the small scale wind turbines. Not only they are able to perform more efficiently but they also out last the smaller counterparts because they are made to spin much slower, thus reducing wear and tear. Although their efficiency could be further increased by adding more blades but the additional weight and cost tends to offset the relatively less gains made in performance.
3 -bladed wind turbines have seen exponential growth over the last decade and have become an icon for renewable energy in today’s collective consciousness. With huge amount of investment and availability of cheaper finance for wind farm, the price of wind has dropped by as much as 30% over the last 8 years.
- Stirling Dish Systems
Solar Stirling Dish System
For arid climates, Stirling dish engine presents a very effective and robust solution. It uses the zero-emission Stirling engine in conjunction with a solar dish. The Stirling engine was conceived almost 200 years ago in 1816 by Robert Stirling. Unlike normal internal combustion engines it does not require any fuel (petrol or diesel). It only requires a hot and a cold source and the greater the temperature differential between the sources, the more efficient the engine runs.
The solar dish collects and focuses solar energy on a smaller area which heats up and acts as a hot source for the Stirling engine. The engine is connected to an alternator which produces electricity. The output can be scaled up by increasing sizes as well as number of units. Such systems have shown a very respectable energy conversion efficiency of 31%. Stirling dish systems are already churning out electricity in countries like Australia, USA and China.
- Multi-Junction (MJ) Solar Cells
Concentration Multi Junction Solar Cells
The highest efficiency (light to electricity conversion) was recorded by stacking solar cells (of different band-gap) on top of each other. This allows each cell to absorb photons of different wavelength range. Developed for space applications, where both weight and volume carry a premium, they have shown a peak efficiency of 44.7%. They are expensive at present but can use concentrated solar power through lenses or mirrors to offset their high cost. Multi Junction solar cells are currently being used in the Mars Rover missions. With a possible theoretical maximum of 87% efficiency with this approach of layered PV cells there is scope for further improvement.
It is due to their high efficiency that a lesser fraction of incident solar radiation converts into heat. This keeps the cells cooler compared to mono crystalline PV. This phenomenon is desirable as the efficiency of the cells has a tendency to go down with the rise in temperature.
- Shrouded and Ducted Wind/Tidal turbine
Venturi Tidal Turbine
Shrouded wind turbines are marketed as having to three times the efficiency of conventional wind turbine. It uses “Venturi effect” to overcome Betz limit (theoretical maximum efficiency of 59% for wind turbine blades). Its potential is not limited to wind and similar designs are being tested for tidal energy, where due to high density of water they generate even more energy for the same size. The shrouds normally increase the efficiency by a factor of 1.4 -1.6.
The best designs are therefore up to 45% efficient overall. Even with such high numbers to boast, shrouded turbines have not made in roads because in stormy condition the drag on them increases substantially. This requires stronger foundations and hence increased cost. With the increased drag on the foundation in stormy condition being a vice for wind source, they are likely to see a faster growth in their underwater version. Applications where the length of the blades is restricted, shrouded turbines can give a higher yield.
- Flat Plate Solar Water Heaters
Flat plate collectors have a simple design that involves copper pipes mounted over an absorber plate covered with glass lid and wrapped with insulation. If selective coating is used on the absorber plate along with high quality insulation and low iron glass than they can be as efficient as 70% (Solar insolation to heat). Flat plate collector is one of the cheapest ways to tap into renewable energy and its payback period is small (depending upon the amount of hot water used). They can be installed on roof tops without looking obtrusive. If new-build houses incorporate them upfront i.e. during the construction phase than they can seamlessly integrate into the building fabric. This can make them even more feasible than in the case of retrofitting. Flat plate solar collectors can also be employed for swimming pools where large body of water has to be heated to a temperature slightly higher than mains water temperature. For such applications, unglazed flat plate collectors present a very cost effective solution.
- Evacuated Tube Solar Water Heater
Vacuum Tube Solar Water Heater
A vacuum tube solar water heater can convert up to 80% of the energy incident on it to thermal energy in the form of hot water. By minimizing the losses due to convection, using a vacuum tube, the collector passes almost all the solar energy received to water/ heating fluid. This functionality also allows it to produce hot water even when the ambient temperatures are below freezing.
They can be used both domestically and commercially (as economizer for industrial boilers). Due to economy of scales, the price for it has dropped substantially over the last decade. Furthermore because of their modular nature of assembly any tubes that break can be individually replaced rather than replacing the whole unit. Although on the outset they seemingly look fragile but good quality heater have shown that the vacuum can last in the tubes for over 25 years. Similarly the retention of vacuum also prevents the reflective coating from degradation.
- Salter’s Duck
Salter’s Duck
In 1974, Stephen Salter’s wave power device became known as Salter’s duck or the nodding duck, although it was officially referred to as the Edinburgh Duck. On small scale controlled environment, the Duck’s curved cam-like body showed that it can stop 90% of wave motion and can convert 90% of that to electricity giving 81% overall efficiency. Although in open waters the expected efficiency would be lesser than what was recorded in a wave tank, but given that there is a global potential of 2 TW in wave energy, it is a technology not to be ignored. Salter’s Duck inspired a new generation of modern devices such as wave attenuators (Pelamis) and oscillating wave surge convertor (Oysters) that are now operating commercially.
The second generation wave devices are currently getting deployed while the third generation devices, that will be even more robust and efficient, are undergoing research. As wave energy convertor operates off-shore they have minimum noise and visibility impact.
- Wood-Burning Stove
Wood Stove
Although it might not look very glamorous in the list of hi-tech machines, but with a combustion efficiency of 89% it is one of the top contenders. Can it be classed in the Renewable device category? The short answer is “Yes”. Wood stove qualifies as a renewable energy device, provided that the wood used for it is harvested sustainably i.e. in a way and in quantities that would enable it to be replenished at the rate it’s used. The carbon- dioxide that is emitted as a result of seasonal combustion from the stove is recouped by the wood growth over the year. It is also one of the cheapest ways to use renewable energy.
There is a huge variety of stoves in the market and some have the ability to swivel so that heat can be spread across different areas in the room. The quality of wood also determines the stove efficiency.
- Hydro-Electric Turbines
Hydro electricity has numerous advantages. They can step-up and step-down the power output in seconds by regulating the flow valves. They can also act as an energy buffer (pumped storage), a feature that is extremely useful when there is surplus electricity in the grid due to the fall in demand. Francis and Kaplan turbines are amongst the most efficient prime movers and can achieve efficiency of 92% under certain conditions. The fact that density of water is almost 800 times more than wind, hydro-turbines with a relatively smaller size can produce massive amount of power. The price for a unit of electricity produced by hydro-electric turbines is cheaper than other fossil fuel based technology. It is no wonder that it is the most productive and widely used form of Renewable energy. It is also worth noting that these hydro-turbines can also be employed in tidal barrage to extract tidal energy.
Also worth mentioning:
Heat Pumps
Some of the available ground source heat pumps have very high Coefficient of Performance (C.O.P) of 5. This implies that 1 kWh of electricity would yield 5 kWh of heat. However, from the efficiency perspective it would be disingenuous to call it 500% efficient as it merely transports energy and does not convert it. It should also be noted that better quality heat source results in better C.O.P. For example ground source is better than underwater source which is better than air source.
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