Solar Integration and the Environmental Impact of Solar Power
Abstract
Solar power, particularly in the form of solar cells, provides one of the most important renewable energy sources for combating global warming by replacing fossil fuel generation and reducing carbon dioxide emissions. However, the production of silicon for solar cells, the most important source of solar power, is extremely energy intensive, and with the current global energy mix this creates a carbon footprint for solar electricity production. Even so, all types of solar power plants are less environmentally damaging than any type of fossil fuel plant. Solar power is intermittent, and when the level of grid penetration of solar power is high, measures must be taken to ensure grid stability. These include accurate weather forecasting, the use of programmable inverters to control solar modules, and the use of energy storage.
Keywords
Carbon emissions; silicon carbon footprint; life cycle costs; solar grid integration; weather forecasting; programmable inverters; energy storage
Solar power is one of the major renewable sources of electric power available on Earth, alongside hydropower and wind power. Hydropower is well-established and has the largest installed capacity of any renewable source. Rapid development of wind power began in the early 1990s, and its global capacity is now large too. Solar power developed alongside wind power beginning in the 1980s, but for most of that period it has been considered too expensive to offer an economical source of electricity. That changed following a dramatic fall in the cost of solar cells; solar capacity is rising rapidly now too. Over the long term, solar power, particularly in the form of solar cells, offers the most reliable and accessible form of renewable power generation. It is available globally, and during the rest of this century it is expected to develop into the most important source of energy.
The value of these renewable resources is their ability to generate electricity without emitting any carbon dioxide, and in most cases, any other pollutants. Global warming is now a major threat to global security and the global economy, so reducing these emissions has become a high priority. Renewable electricity generation is one of the most important tools available for achieving this.
In this context, solar power is not entirely clean. Production of silicon used to manufacture solar cells is an energy intensive process, and much of the energy used today to prepare pure silicon comes from power stations burning fossil fuel. Other solar photovoltaic (pV) cell semiconductor materials also have significant energy costs. In addition, solar thermal power plants require high-technology components such as mirrors and steelwork, and these require energy to manufacture. However, both types of power plants are responsible for fewer emissions over their lifetimes than any type of fossil fuel plant. The materials used in some solar cells are also hazardous, and that must be taken into account when considering the environmental impact of solar power. Overall, however, the environmental benefits far outweigh the risks.
Solar plants have other environmental effects. Land usage is high compared to fossil fuel plants, and solar thermal plants often need water, a problem in the arid regions to which they are best suited. There is also an operational issue with solar power, a consequence of the fact that solar energy is only available intermittently. Solar power can only be harvested when the Sun is shining. When the Sun goes behind the horizon, solar power plants stop producing power. This means that a solar power plant cannot provide a reliable supply of electric power on its own. This intermittency is a problem that rises in importance as the amount of solar power connected to the grid increases. Techniques to help mitigate intermittency problems are being developed alongside other advances in solar power generation.
The Environmental Impact of Solar Power
From an environmental impact perspective, solar thermal and solar pV power plants need to be considered separately. Solar thermal power plants are similar to traditional fossil fuel-fired power plants with regards to their use of steam turbines and associated equipment to generate power. However, solar plants use a large solar collector field instead of a coal boiler to collect heat and raise steam. This collector field is of significant size for a large power plant, using between 2 ha/MW and 5 ha/MW. Further, if land is used for solar energy capture it is difficult to use it for any other purpose, unlike wind power, where land can be used for crops or animal grazing. On the other hand, it is possible to build solar plants on poor quality land; the best sites are likely to be in desert regions where insolation is high. This makes it less likely that solar plants will be competing with other land users.
Of greater concern is water usage. A solar thermal plant requires some form of cooling in order to condense steam as part of the power generation heat engine cycle. This is normally provided by water cooling. A typical solar thermal plant with a recycling water cooling system requires around 2–3 m3 of water for each megawatt-hour of power generated. Once-through cooling systems can use more water, although overall water loss through evaporation is generally lower. The alternative is dry cooling, which can cut water use by 90%, but at a cost to efficiency. However, dry cooling can be less effective at high ambient temperatures, conditions that are likely to be common where solar thermal plants are sited. Because the best sites for solar thermal power generation are arid regions where there is low rainfall and a generally dry climate, water use can present significant difficulties. It may be possible to bring water from less arid regions, but that would push up the cost, possibly making a plant uneconomical.
A large solar thermal power plant will affect the habitat upon which it is constructed. However, the effect may be relatively benign unless the site is also the habitat for an endangered species. The power plant collectors will offer additional shade that was not previously available, and this could prove beneficial. A potentially more serious issue relates to the effect of large solar tower plants on birds. These power plants function by achieving high levels of solar concentration, much higher than in other types of solar thermal plants. In essence, they create high temperature beams of sunlight that can injure or kill birds that fly through them. The issue is emotive and the severity of the problem is still under investigation, particularly in the United States, where the largest plants of this type have been built.
The construction of a large solar power plant, thermal or pV, involves local disruption of traffic movements and noise. There is also the possibility of potential spillages from the heat collection circuits and the steam generation components of a solar thermal plant. However, these factors should not represent a serious risk at a well-managed plant.
Like solar thermal facilities, utility-scale solar pV power plants have high land usage requirements. Typically a plant will use between 1 ha/MW and 4 ha/MW, slightly lower than for a solar thermal plant. As with a solar thermal plant, it is unlikely that the land can be used for any other purpose. This should not be a problem if plants are sited in areas where the soil is poor and has little agricultural value. Aside from the disruption during the construction phase, a large solar pV plant should have little local impact.
Land use is much less of an issue with smaller solar pV installations as many of them are placed on rooftops. These installations can often be unsightly, but in most instances they are not visually intrusive. The development of architecturally designed rooftop panels can certainly improve their appearance, as can the integration of solar panels at the design stage in building construction. Some modern developments include solar panels as part of the structure; these usually stand out less, and may even prove visually appealing. There have been proposals to incorporate solar cells into windows too. Based on luminescent collectors, these could potentially make solar generation invisible. However, this technology still has a long way to go before it can be deployed commercially.
Other than land usage, solar pV plants have little impact on their environment while operating. Utility plants have mechanical systems to allow the modules to track the Sun, and these could cause minor oil spillages. Rooftop systems, however, have no moving parts. Problems could potentially arise when a plant reaches the end of its life. Decommissioning these plants involves recycling large quantities of semiconductor material. Regulatory approval of many modern pV plants requires the manufacturer of the solar pV cells to take them back for recycling at the end of the life of the plant. This is particularly important for some thin film cells that include environmentally hazardous materials.
Life Cycle Cost
While solar power plants produce electricity without the emission of carbon dioxide, they are not emission free. The components of both solar thermal and solar pV power plants are made using energy intensive processes. These include the production of steel, glass, and specialty metal components for the heat absorbers in a solar thermal plant, as well as all the components of the steam turbine generator plant. Significant quantities of concrete—an energy intensive product—are likely to be used in both types of solar plant. In addition, there is one outstanding life cycle cost for silicon pV plants, the energy cost involved in the production of the silicon.
The actual cost in emission terms of producing silicon depends on the mix of technologies that are generating the electricity that is used to manufacture the silicon. For the situation in Europe, estimates suggest that the lifetime carbon emissions of a solar pV power plant are between 20 gCO2/kWh and 80 gCO2/kWh. This is around 10 times less than the emissions from a fossil fuel power plant.1 These emissions are equivalent to an energy payback time for a commercial plant in Southern Europe of 0.7–2.5 years. The life cycle cost for cadmium telluride can be of the same order of magnitude. This compares to lifetime carbon emissions from a coal-fired power plant of around 900 gCO2/kWh, and emissions from a natural gas-fired power plant of 400 gCO2/kWh. Solar thermal power plants show better lifetime performance than solar pV plants, with average lifetime emissions of around 30 gCO2/kWh.
These figures depend in part on the lifetime of the actual power plant. The assumed lifetime of a solar pV plant is 25 years. If this was extended to 50 years or even 100 years, a lifetime that may be possible in the future, then the lifetime emissions would fall. In addition, as the world switches from fossil fuel to renewable, the lifetime emissions will fall because renewable sources will contribute more of the power that is used during the manufacturing processes.
Solar Integration
Solar power is by its nature both intermittent and unpredictable. The source is intermittent because the Sun only shines during daylight hours. This has to be taken into account when building grid-connected solar stations.
The change from night to day and back is highly predictable, as are the seasonal changes as the Earth moves around the Sun. This predictability makes it relatively easy for grid controllers to manage the broad cycle of solar power generation. However, on top of the diurnal variation in solar intensity there is a random variability resulting from the weather. Clouds can reduce solar intensity significantly and rapidly, and this can lead to big changes in output from solar power plants.
The variability of solar output is not an issue when there is only a small amount of solar power being injected into a grid system compared to the overall grid-connected generating capacity. All grids are designed to cope with variations in both demand and supply. However, when the level of solar penetration as a proportion of total production on the grid rises beyond a certain point, the solar input can become a problem because it behaves differently to other sources. This is complicated by the fact that solar power must normally be dispatched when it is available, so grid operation must be designed around maximum use of solar—and other—intermittent renewable sources.
The variations in solar power output from solar plants connected to a grid system can be substantial. Across the nations of the European Union in 2011, the average amount of solar power available for dispatch during high summer reached close to 10,000 MW. In the depth of winter it was only 2200 MW. Short-term variations can be massive too. This is particularly noticeable at sunrise and nightfall when the output from solar plants ramps up and down very rapidly. Meanwhile, the arrival of cloud cover can reduce solar output by as much as 50% over the area affected. In 2011 in Germany, the maximum hourly change in pV output was 10,300 MW; in Italy it was 7200 MW. Grid operators have to be able to manage these changes by using other plants to support the solar output.
At a grid-wide level, resilience in the face of large amounts of solar power input depends upon factors such as the amount of energy storage or fast-acting grid capacity that can step in or back out as solar input rises and falls. Grids that have significant amounts of hydropower can be particularly resilient because hydroplants can start up and shut down very quickly. At the grid line level the problem often manifests itself as one of voltage stability. This is frequently caused by too much power being injected into the distribution system to which many rooftop solar systems are connected. The excess power then feeds back from the distribution system to the transmission grid through the distribution/transmission system transformer substations. In most conventional grids power is only supposed to flow from the transmission system to the distribution system, not in the other direction.
The problem of solar input may not be grid-wide, however. It can cause local problems too, on a single grid line, for example. Again, the most pragmatic way of determining the effect is through grid voltage. When this starts to fluctuate as a consequence of solar input, the solar penetration is too high. At what penetration level this occurs depends on the situation. It might start at 15% solar penetration if the grid line is long and has little voltage and frequency support; on a stronger line it might appear until penetration reaches 95%.
There are mitigating factors that help accommodate solar power at all levels of the grid. One of the most important factors is the coincidence of peak solar output with the hottest period of the day. This occurs when there is the greatest demand for air conditioning, so solar output will, at least partially, match a particularly hot summer day’s demand and alleviate the need for grid controllers to bring expensive peaking power plants online.
Another important means of managing solar power is by using accurate weather forecasting. Modern forecasting techniques can provide a good level of reliability, and forecasts are used routinely in most grid control centers today. Good day-ahead and hour-ahead forecasting, particularly if used to predict the weather over each solar plant connected to the system, can make it easy for operators to anticipate changes in solar power output and plan for other sources to either back out or come online as needed.
New technology can also help manage solar input into the grid from solar pV systems. A device that is at the forefront here is the smart inverter. As noted in Chapter 10, inverters form the interface between the solar pV system and the grid. A modern inverter can be programmed to modulate the output of the solar system it controls. In the past this has been carried out simply by switching the pV output on or off. However, new inverters can carry out partial “curtailment” by reducing the output rather than shutting it off completely. If the installation is able to communicate with the grid controller, this can be controlled centrally, allowing a fine level of management.
The final grid tool for managing intermittent sources such as solar power is energy storage. Energy storage can be implemented at the transmission grid level, at the distribution grid level, or at the level of individual pV installations. There are many storage technologies that can be used, but the most important for solar pV are batteries, often installed at a local level. These are fast acting and can either absorb power or supply it, depending upon the conditions. At the grid level, pumped storage hydropower is also effective. Energy storage is considered expensive, but it is likely to form a much more significant component of national grids as renewable generation grows.