This article explains the advantages of solar energy compared to other renewable energy sources.
The current trend of increasing attention to renewable energy among policy makers, business community, and the public, tends to portray it as a unified field that beats traditional energy sources on all fronts. The recent roadmap, published by a team of researchers from Stanford University, also envisions a transition to “100% of wind, water, and solar”, as if they were indistinguishable. While it is true that the ultimate aim of the short- and mid-term policies targeted at renewable energy support is to diversify energy portfolios, rather than to create a mono-energy source system, there are significant differences between the main sources of renewable energy.
We have already written about the distinctive features of renewable energy before. First of all, the correct way is to call it not just renewable, but variable renewable energy, or VRE. The reason is simple. It is more important to distinguish renewable energy from the point of view of energy supply system, rather than simply based on the generation source. The most important difference of renewable energy is that it is inherently unstable, as opposed to traditional, “baseline” energy generation.
Suppose you own a coal power plant. Once you’ve set it up, having spent a considerable amount of capital to meet the upfront costs of building the facility, you can operate it steadily and without major interruptions. Having reached the full capacity in terms of output, you will see the breakeven point fastly approaching, and the average costs gradually falling down to a minimum. In other words, provided there is a stable demand for electricity, a coal power plant (or, for that matter, a nuclear one) can generate relatively cheap electricity in a stable long-term way. This allows for a greater stability in electrical supply and ensures the minimal necessary demand is always balanced. The cost of this, of course, are heavy pollutions and environmental unsustainability, as can be clearly seen from the picture below (a bit old, but still relevant).
On the contrary, when it comes to renewables, there is a very limited possibility to predict and control the output of solar, wind, and water energy generating objects, as opposed to traditional generation. Since the supply of renewable energy depends on the weather and climatic conditions, it requires a far more flexible system of demand and supply management. Many policy initiatives in the field of renewable energy support, and many energy tech applications are designed precisely to enable such flexible coordination.
However, here the differences between various sources of renewable energy come into play. Water, wind, and solar energy generation differ in terms of their predictability, their efficiency, and their costs. Here’s how.
Cycles and Scales
The first point to be considered before turning to actual comparisons of different renewables is the fact that all energy sources have their natural reproduction cycles. Even fossil fuels are not an exception, although their cycles of reproduction exceed the lifecycle of a human being (not to say the renewables’ lifecycles) by several orders of magnitude.
The following table, adopted from a research report by Clean Line Energy, summarizes the differences in the timescales of the natural cycles of renewable energy sources.
As can be seen from the table, all renewables are different in terms of the temporal scales of their lifecycles. The first conclusion that can be drawn from it is that it is possible to achieve a sufficiently diversified energy portfolio, based on the renewable sources alone. Renewable energy sources that are more prone to temporal fluctuations in the short-term can be supported by more stable generating objects that still use renewable energy.
The second important issue the table shows relates to the requirements of flexible balancing of electricity supply and demand. Such variable sources as solar and wind energy require very flexible patterns of supply and demand management, but can also provide for a greater flexibility if they are included in the energy portfolio of a community or a country.
Finally, roughly speaking, the table can be read as a snapshot of some broader correlations: the shorter the scales of temporal variability of an energy source, the more flexible it is, and the less upfront investments are required to install such a facility. While biomass is an exception to this rule, all other energy sources can be ranked in such a manner (in fact, it does not fit to the picture also because it is not clean).
Thus, geothermal and hydropower plants require heavy capital expenditures; wave- and tidal power facilities occupy the middle, and solar and wind energies are the cheapest ones in terms of the upfront costs. The problem with waves and tides is that, while being generally in the “golden middle” in terms of costs/output stability ratio, they are very site-dependent.
Tidal power is the only source of renewable energy that is independent from the Sun, while the others are indirectly related to it one way or another, including ven fossil fuels and biofuel. On the contrary, tidal power is embedded into the nature of the Earth-Moon system interactions.
Essentially, tides occur because of the movements of the Sun and the Moon, as well as because of the Earth rotation effects, and the effects of landscape. The gravitational forces exerted by the celestial bodies create motions or currents in the oceans of the Earth. The sea level changes as masses of water move horizontally due to the gravitational effects. As the sea level increases, water from the middle areas of the oceans moves closer to the shores, thus creating a tide.
The tides are quite predictable and occur in according to the three interacting cycles:
- A half-day cycle caused by the rotation of the earth within the rotational field of the moon results in tidal movements every 12 hours and 25 minutes.
- A 14-day cycle based on the superposition of the gravitational fields of moon and sun.
- Interaction of the gravitational fields of sun and moon at new and full moon result in maximum spring tides.
- Minimum neap tides occur at quarter phases of the moon, when the sun’s force of attraction cancels out that of the moon.
All these cycles are highly predictable, and so is the variability of the tidal energy output. The following picture illustrates the distribution of tidal phases:
These periodical movements of tides can be exploited to produce electricity. Currently, there are two main technologies. The first one is to harness power through dam-like structures that trap rising waters on one side and release it back to the other turbines that spin to generate electricity. The second one is tidal stream technology that harnesses fast-flowing currents to spin turbine and generate electricity. The former is best known, while the latter is only beginning to be tested commercially.
The main advantage of tidal power is that tides will be there as long as there are celestial bodies of the Sun system. They are thus renewable, and much more predictable than wind and solar power. However, in the case of tidal electricity, location is everything.
First, depending on the changing positions of the celestial bodies, the magnitude and character of the tidal motions also varies. The effects of the Earth’s rotation and local geographies of the sea levels and coastal lines have an impact on the availability and intensity of tidal power.
Second, tidal power plants are very site-dependent, and the number of places where they can be constructed is very limited geographically, as opposed to the other renewable energy sources. The following map, created by B.C. Energy, illustrates this point very clearly:
The availability of sites where a tidal power station can be constructed is limited by the geography of the sea and coastal lines, because the tides of sufficient range and flow velocities are to be found only in certain places. The other problem with tidal power is its high cost that requires high upfront investments in the construction of the tidal facility. The same can be said regarding the various forms of hydropower, although it is much less site-dependent, it requires even more initial investments to create artificial lakes at which the hydro-electrical plants are based.
Wind and Wave Power
Winds are created by the Sun’s heating of the Earth and the latter’s rotation. Wind power is exploited by the means of Horizontal Axis Wind Turbines (HAWT), which represent 90% of the world’s wind turbines in use (there is also an alternative, Vertical Axis design that comprises the remaining 10% share). There are also smaller wind turbines in use by individuals.
HAWTs have large angled propellers with blades that catches the wind. As the wind passes through the blades, it causes the entire blade assembly (the rotor) to spin around the central nacelle on the top of the tower. The nacelle is a complex housing, in which a gearbox is located. The gearbox converts the incoming rotational force with a low speed into a high-speed outgoing rotational force that is powerful enough to run an electrical generator that is also located in the nacelle.
While generally cheap and widely available, wind power is the least predictable of all of the variable renewable energy sources. Because of the variable nature of the wind, grid operators are compelled to use day ahead forecasting to optimize the use of available power sources next day. They also rely heavily on weather forecasting to predict the likely wind energy output. The picture below presents an example of the day ahead prediction and actual wind power, evidencing a rather strong correlation between the two:
Wind power also has other limitations. It is highly intermittent and non-dispatchable, since it depends on many factors that have an important impact on its output. First, location does matter, although not as much as in the case of tidal power. Second, such things as wind speed, air density, and the characteristics of the turbine (among others) can cause significant variations in the output of wind power generators. The speed of the wind is one of the most important factors, since, depending on the turbine, it must be above 3.5 m/s in order to generate electricity, but below 25 m/s, otherwise it would damage the turbine.
Wave energy largely depends on wind, and that’s why the two can be considered together. In general, the power available from waves tends to follow that available from wind, but due to the mass of the water is less variable than wind power. The fluctuations of waves energy are different, as waves in deep water lose their energy and by this smooth out only slowly and therefore can travel long distances. Wave energy, however, is subject to cyclic fluctuation as well, dominated by wave periods and wave heights. As a result of these fluctuations, the power level available from waves varies daily and monthly, as well as seasonally.
There are two primary sources of geothermal energy: radioactive decay and the primordial heat of the Earth that was created during its original formation. In the former case, the process of decay of certain radioactive elements (like uranium-235 or thorium-232) occurs naturally in the ground below the Earth’s surface. As a result of this process, a lot of heat is generated, that can be used productively. Since the Earth’s interior has only decreased its temperature by a few hundreds degrees over the entire period of its existence, geothermal energy is practically inexhaustible, and the process of radioactive decay is ongoing anyway.
In the latter case, the solid outer layer of the Earth’s surface insulates us from the heat that was produced in the process of the plant’s formation. The primordial heat continues to flow from the interior of the Earth to its surface through the slow conduction of solid rocks, and heat transport fluids like water and magma. It can also be usefully exploited.
To do so, one needs to find a large source of available heat, put it into a reservoir to contain it, and lock it in there using a barrier. Finally, there must be some kind of carrying agent, for example, a fluid to transfer the heat.
The reservoirs are usually rock units with high permeability and temperature. Once such a hot unit of rock is surrounded by impermeable rock layers, the latter can function as barriers and contain the heat. The extraction of geothermal is carried out by means of drilling into the reservoirs. The conventional way of extracting geothermal power is implemented in the locations where the rock is porous, and there is hot water inside. Such locations are usually found in the areas where magma has poked through the continental crust and created convective circulation of groundwater.
Geothermal power has many advantages, including its very stable and predictable nature, as well as minimal operating costs. However, the initial capital costs are significant, being sometimes up to $4 M per 1 MW, depending on the size of the power plant and local geography. Over 50% of the costs are absorbed by drilling. Moreover, geothermal power is somewhat site-dependent and, most importantly, can be a very risky investment, because after spending millions on exploration, the resources found can be unfit for exploitation.
We have already written about how PV solar stations work and what is the nature of the photo-effect, so let’s concentrate on its advantages as a source of renewable energy.
Solar PV plants can operate for years without incurring much of operation and maintenance costs, so that the O&M costs are extremely low as compared to conventional power technologies.
In grid-tied PV systems the electricity produced can reduce or eliminate the use of grid electricity during peak hours of operation (during the day). This advantage requires a time-of-use meter, which may not be available to some users. Grid-tied PV systems also reduce the amount of transmission losses that occur as a result of transmission of electricity over long distances. They can also reduce or eliminate completely the use of grid electricity during the peak hours.
The other advantages of PV solar energy can be listed as follows:
- The sun is a clean, renewable, energy resource that is proven and increasingly cost competitive, as the costs of solar panels steadily fall down, and more research and development efforts are put into the field of solar photovoltaics
- Increased use of solar energy builds energy security, reduces greenhouse gas emissions, and moves us toward a sustainable energy future
- Using solar PV systems help reduce peak loads, postponing or preventing the need for additional baseload energy generation and distribution infrastructure (hydroelectric dams, coal-fired power generation stations, and underwater electrical cables)
- Solar requires no fuel or moving parts, makes no noise and produces zero emissions with minimal maintenance.
- In remote sites, solar PV competes aggressively with the costs of electricity derived from conventional sources and areas requiring extensive power line construction may find solar PV to be more cost effective.
In sum, solar energy is the best investment choice among the sources of renewable energy. It is not as heavy in terms of the capital costs as tidal and geothermal (and much less risky); it is simple, but, unlike wind and waves, quite predictable. It is also much less site-dependent, although it requires considerable amounts of free areas. As the industry develops, the costs of solar panels, as well as capital costs per unit of energy will continue to fall down, making the investment opportunity even more interesting. The concluding pictures provide a few snapshots of the lay of the land in the solar photovoltaics over the recent decades:
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