Solar and Thermal Power Systems

Steam has been used for many centuries to enhance mechanical work. One of the most popular machinery utilized to convert the steam power to mechanical energy has been the steam locomotive engine. Most modern steam turbines perform similar functions but at a much higher energy conversion rate. Most steam turbines are often used due to their high efficiency when turning steam energy into the vital kinetic rotational energy. The rotational energy can be then utilized to drive electricity generators or various processes that need mechanical energy for its operations. According to history, the steam that was needed for these processes was acquired from burning fossil fuels such as natural gas and coal ( Avila-Marin, 2011) . The steam that is generated by solar radiation is similar to the steam obtained by burning the fossil fuels and heating water. The conversion principles of solar heat to electrical and mechanical energy are basically similar to the techniques used in combustion systems. Concentrating solar thermal systems is best suited to attain higher efficiencies and high temperatures under high pressure while meeting the requirements of large-scale turbines that need large amounts of high-quality steam. 

The solar energy is received and converted to heat via the collector system. The heat is then transferred through the thermal fluid to a storage and then to the boiler where steam generation takes place. The steam is transferred to the turbine in its heat engine. The steam is converted to mechanical energy, and some heat will be rejected. If the desired output is electric energy, the mechanical power is transferred to a generator which converts it to electric power. However, one of the key challenges in the system is the energy losses. The efficiency of solar collectors reducing with an increase in the operating temperature ( Hu et al., 2010) . Also, the heat engine efficiency increases with higher operating temperatures. The operators maximize the solar power output at the optimum conditions. No machine operates at a 100% efficiency. Plant plate temperatures do not provide efficient temperatures to run heat engines. Therefore, evaculated tubular collectors or concentrating collectors such as parabolic systems are desired. The paper will analyze solar thermal power systems, its financial requirements, and its future needs. 

Solar Thermal Power Plants 

Solar radiation is concentrated to compensate for solar radiation attenuation on its path on the earth’s surface. It is acquired from 63.2 GW/ from the sun to about 1kW/ on the earth’s surface ( Ma & Turchi, 2011) . Higher temperatures can only be achieved with higher concentrations to convert solar radiation to thermal energy. Some of the power plants include: 

Parabolic Trough Collector Systems 

Parabolic trough technology is the most popular among all the utility-scale thermal power systems. The temperature potential of this kind of concentrating collectors is quite high and can offer output fluid temperatures that go as high as 500 0 C. The parabolic trough acts as the linear focus collector and is made up of cylindrically curved parabolic mirrors that reflect solar radiation or sunlight to a tubular receiver that is located on the focus line of its parabola ( Jamel, Rahman, & Shamsuddin, 2013) . A fluid that is heat absorbent is placed in the tubular receiver, and it transfers the heat to a boiler or any other device that produces steam. 

The above power plant is located in Kuraymat, Egypt. The solar thermal power plant has a solar aperture of about 130800 . It electricity generation estimates is about 34000 MWh/year and has been operational since 2011. It contains parabolic mirror rows that are placed parallel to each other either on the east-west axis or north-south axis and change position while tracking the position of the sun in the sky. There are merits and demerits of both orientations according to the energy needs and location. The tubes are specifically the design of absorbing solar radiation and transferring it to the heat exchange fluid in the tube ( Hu et al., 2010) . The fluid is then pumped via absorber tubes that are connected in parallel and series. Some plants use an insulated storage tank to allow power generation even when solar radiation is unavailable or intermittent. It is intermittent when there are dense cloud covers and unavailable during evening hours. The heat exchange liquid is transferred to the storage tank, if it has achieved the required temperatures, and pumped to heat exchangers that transmit the heat to water for steam generation that is transferred to the steam turbine for electricity production. 

The biggest parabolic trough parabolic solar thermal power plant is known as Solar Electricity Generating Systems (SEGS) and was developed in Southern California, USA. It produces about 354 MW. However, it is surpassed by the Ivanpah Solar Power Tower System that produces 377 MW. The three main solar thermal power plants are located in the USA and have a power production that ranges from 250 MW to 354 MW. The next twelve biggest solar thermal power plants are located in Spain and have power production estimates ranging between 100 MW to 200 MW ( Jamel, Rahman, & Shamsuddin, 2013) . Parabolic trough solar thermal power systems are one of the most successful renewable energy over the past two decades. Among all solar thermal energy systems, parabolic troughs have the lowest cost solar power and have huge potential for further reduction in its cost. 

For instance, the California Mojave Desert has nine parabolic trough plants that produce about 350 MWe and have been operational for about two decades. The power plants produce enough power to meet the electric power requirements of the residential city with a population of 250000 residents. Solar thermal electricity production has demonstrated remarkable availabilities and has been reliable through the energy crisis in California between 2000 and 2001 ( Kuravi et al., 2013) . During the incident, it had about 100% availability during the solar peak hours. Although it has the least cost solar power choice, it is about twice as expensive as the power generated from fossil fuels in the US such as coal. 

Central Receiver Systems-Power Tower 

Different to linear concentrating systems such as troughs, that reflect sunlight to the focal line, central receiver systems transfer the concentrated light to a central receiver which is remote. An example of such systems is the solar power tower system that is made up of several tracking mirrors known as heliostats that are placed in the field surrounding the main external receiver that is placed on a tower ( Ma & Turchi, 2011) . These systems can attain higher levels of concentration in comparison to linear systems. The concentrated radiation is then utilized as heat in steam production, which drives the steam turbines to produce electricity. Also, the produced thermal energy may be stored in various storage points in the form of molten salt. 

The above solar power plant is known as Planta Solar 10, and it is located in Spain. It generates about 23400MWh/year and has been operational since 2007. The sunlight is then concentrated at the top of the tower which is 115 meters high. The central receiver systems are basically large-scale power plants that are created to power the steam cycle. The receiver is placed in a central position that gives it a universal advantage for energy collection at one specific location, therefore, saving costs, and transport networks. However, the since the central receiver is fixed, it limits the amount of light that can be collected. Furthermore, the heliostats are placed at an angle to the direct rays of the sun. Therefore, the amount of light that may be collected is often lower compared to the parabolic concentrators ( Avila-Marin, 2011) . To attain the required light concentration efficiencies, the amount of land under the heliostats should be increased. The main demerits of such power plants are their land use, higher capital costs, and higher environmental impact. There is huge potential for building large-scale central receiver systems in flat, arid areas, and deserts which receive plenty of sunshine and the value of land is lower in comparison to other industrial applications. 

Southern California on its border with Nevada has three towers that are located in an area about 2.5 million square meters in size. Under the optimum conditions, the system can generate about 377 MW. Solar power tower models have been operational for many decades for generating steam and other industrial applications. Plants that are used for research purposes produce tens of MW. The solar power system, known as Ivanpah, was the first and only solar tower system globally that produces more than 20 MW. Although there may be plans in various countries to build similar plants globally, there is limited experience in such systems ( Kuravi et al., 2013) . After high-quality steam is produced at the tower, it is then pumped to the generation station located at the tower's base. Also, the remaining parts of the solar power tower plant are not different from other conventional forms of electricity generation. 

The heliostats track the sun’s movement. Each device is accurately positioned by the computer control system to direct the sun's beams to the tower that has a thermal receiver. The impact of many heliostats that are reflecting sunlight to some common points produces the combined power of sun's rays, therefore, producing high-temperature thermal energy. The thermal receiver contains molten nitrate salts that absorb the energy reflected from the heliostats. The heated salt is later utilized to boil water to steam. The steam is then directed to the steam turbine to generate electricity. There are some countries that have had more history and experience with the solar thermal tower technology ( Xu, Li, & Chan, 2015) . The USA and Spain are the two main countries in this technology. Also, there are various countries that have plans to build or are operating power tower systems that produce more than 10 MW. The USA possesses the biggest solar power tower plant globally and has the experience of operating such systems. Spain has three solar power tower plants that produce more than 10 MW each, and they have plans to construct three more plants. 

Solar Updraft Towers 

Solar updraft for producing electricity was initially conceived over a century ago. Various prototypes were developed over the past century, and some plants have been operational. They vary in scale and size. The largest solar updraft towers produce tens of KW of power. Their towers are often hundreds of meters tall such as the Manzanares tower in Spain which is 194 meters tall and produces 50 kW of electricity. 

Figure 5 showing a solar updraft tower. 

The solar updraft power system depends on the stack effect as its principle of operation. The density variation in the air because of humidity and temperature differences causes the air movement. Solar radiation is utilized to increase air temperature at the tower's bottom, therefore creating a density gradient that creates an upward air movement ( Bradshaw, Cordaro, & Siegel, 2009) . The solar induced wind can rotate some wind turbines placed within the limited space inside the chimney. The electricity output of the solar updraft system is determined by two design variables: the height of the updraft tower and the solar concentration area. The solar collector is often depicted as a greenhouse-like structure that encircles the tower at its base. The commercial size collectors are built to an area of about 5 square miles. The greater the high of the tower, the bigger the pressure gradient because of the stack effect and its output is used to produce wind power. Although some proposed system is about 1500 meters tall, the common system heights range between 100 meters and 200 meters. Various proposals for large-scale solar updraft have been presented. However, the implementation of such systems is hampered its high commercialization costs. 

Dish Sterling System 

The Stirling or dish system generates power through the parabolically arranged mirrors that reflect the sun’s rays to a focal area known as the receiver. Therefore, it creates a gas chamber which is attached to a piston and drive shaft. The piston causes a kinetic motion on the drive shaft that powers a generator that produces electricity which is distributed to the national grid. The Stirling system is comprised of two main components: The Stirling engine and the solar dish. The solar dish is just as set of mirrors or a parabolic mirror while the Stirling is made up of a closed cycle engine that is silently operated by any heat source ( Xu, Li, & Chan, 2015) . The Stirling engine efficiency has a similar maximum theoretical efficiency as any other engine technically referred to as the Carnot cycle efficiency. 

Financing Solar and Thermal Power Investments 

There is a common belief among most stakeholders in the solar thermal industry that the financing alternatives or options do not affect the result and should not be included in the economic analysis when evaluating different power technologies. The assumption is true if the different alternatives have similar funding structures and have similar outcomes. However, if one investment is not capital intensive and the other is capital intensive, or when one power investment has low-interest rate financing or a grant is offered for only one technology, then the investment or financial structures will have a profound impact on the outcome ( Salazar et al., 2017) . Most conventional power plants that are fossil fuel dependent can purchase a continuous series of payments over the lifetime of the power plant, but the solar thermal power plants are required to finance their "fuel needs" via intensive capital investments at the initial phase of the project. 

Therefore, the solar equipment investment should be repaid through interest payments and principal on loan during the lifespan or operational time of the power plant. In the popular parabolic trough type plant, the amount of land or solar field accounts for about 50% of the initial investment in the power plant. Because of the perceived and technological risks, the financing and interest rate costs for the solar power investments to be quite significant. Due to the risks involved, the solar power projects investments are quite sensitive to schedules and financing conditions, and it can change drastically with a simple variance in the financing structure or project ownership ( Reddy et al., 2013) . Other than the financing cost penalty, the financial investment in the land or solar field are often taxed differently than investments for fossil fuels. Therefore, solar power investments face inequity in both financing and taxes as well. Favorable tax policies should be formulated to eliminate the financial burden on solar power investments. 

The lifespan of solar power plants may be compared to the typical or conventional power plants, and they have a lifespan of about 25-30 years. For the conventional fossil power plant, the main risks revolve around significant fuel price changes in their lifespan should be evaluated. On the other hand, for a solar thermal power plant, after it has been built, "solar fuel" is free, and it faces less uncertainty at the cost of fuel over its lifespan. Another key challenge is that most third world countries where solar thermal power generation is feasible, they often have inadequate budgetary allocations to fund the capital-intensive nature of solar thermal power plants ( Salazar et al., 2017) . The poor credit rating and financial resources of developing countries may be given a boost through private funding as long as the legal and political situation of such countries is favorable for foreign and local private funding. There has been a deregulation of the global power market, and it has changed the financing of power projects and the future research and development of the technology. 

Renewable power projects have three key sources of capital: grant, equity, and debt financing. Equity investments involve buying or purchasing ownership of the solar power project. Debt investments are where financial institutions offer a loan to the project. For example, when a person buys a house, the mortgage is the debt while the person purchasing the house is an equity investor. Because most solar thermal power projects cannot compete favorably with typical fossil power technologies, various organizations have provided grant financing to help bring down the capital investment of the non-economic segments of the solar power project ( Reddy et al., 2013) . Some of the grants often account for economic, environmental, and developmental externalities that are often accounted for during the conventional competitive project selection. 

Financing and Ownership Structures 

There are two key ownership and financing structures that are available to finance power projects: Project finance and corporate finance. Project finance involves private ownership whereas corporate finance involves an investor-owned utility ownership. Corporate financing may also be known as equity or internal financing, and it involves the use of general assets of the organization and corporate credit, often a utility, as its basis for collateral and credit. Utility ownership has proven to be quite attractive for most capital investments in solar thermal power projects because it can save costs ( Almsater, Saman, & Bruno, 2016) . It has a partial offset involving specific project risk to the corporate/utility portfolio, hence, ignoring the differences in risks that may be associated with the different projects. Also, the organization’s overall credit rating is utilized to estimate the equity and debt costs instead of projecting certain capital costs in specific standalone project finance model. It has a better credit standing, therefore, less restrictive loan covenants, lower interest rates, and increased debt amortization periods. 

Nevertheless, because of the high capital investments, a majority of the utilities in the developing countries may be insufficient to provide adequate corporate finance resources despite the merits of corporate financing. Therefore, the concept and principles of project financing were developed. Project financing may be explained as the project arrangement of credit, debt, and equity enhancements for building or constructing certain facilities in the capital-intensive industries where the lenders can base the credit appraisals according to the estimated cash flows emanating from the project instead of the credit or assets of the investor ( Powell & Edgar, 2012) . The reasoning behind such a move in project financing is to shift the focus of funding solar thermal power projects according to the credit rating of the country affected to the advantages of the investment itself. 

Project finance is the main funding structure preferred by most independent power producers (IPPs). However, there exists a common confusion between the terms “project finance" and "independent power producer." IPP include the whole project environment such as the operation of the facility, project financing, construction, and planning. Nevertheless, because most IPP project often uses the financial structures involving project finance, it becomes a bit challenging to differentiate the two terms ( Powell & Edgar, 2012) . 

There are various characteristics of project financing: off-balance sheet financing, cash flow related financing and risk sharing. Cash flow related lending is where the lenders evaluate the credit rating, and financial feasibility of a project take care of its yearly operation and maintenance debt services and costs along with adequate cash flow. Hence, a lender who may have a long-term credit exposure is often focusing on the capability of the investment cash flow to repay or service the loan in the whole loan life cycle ( Montes et al., 2009) . In risk sharing, it offers a major incentive for most stakeholders because of the project financial undertakings that are allowed to stand alone such self-financing entities that the stakeholders can benefit from its revenues while they can be isolated from its failures. The financing allows the potential investors to shift certain risks to the lender of the project debt. On the other hand, off-balance sheet financing has a different approach. The key goal of the IPP projects is to come up with project finance borrowing that will benefit the private investor while being entirely nonrecourse to other business activities of its parent company. Therefore, it will not affect their balance sheet or credit standing. 

Future Needs and Trends of Solar Thermal Power 

Due to the fluctuating fossil fuel price and increasing energy demands for domestic and industrial consumption, the world may be forced to focus on renewable energy. Some of the countries that have made remarkable steps in solar thermal power generation are Spain, the USA, China, and Morocco. In 2015, they opened the biggest solar concentrated power plant in Ouarzazate in the Sahara Desert. The solar thermal power project is made up of four plants that can generate about 580 MW of electricity ( Hübner et al., 2016) . The projection means that the plants can power more than a million homes in Morocco. The Noor 1, phase one of the solar power project, generates 160MW. The magnitude of the project was a remarkable step for the solar thermal power industry and Morocco’s energy demands and transition plans. It is one of the few middle income earning economies that have put in place robust renewable energy policies and plans. 

By 2020, Morocco has set ambitious plans to supply about half of their homes with renewable energy. For the country to attain such targets, they have come up with regulated policies regarding grants, public investments, and loans. After the concentrated solar power plant was launched, it meat about 10% of the country’s energy needs. Currently, photovoltaic systems are the most popular solar power systems in the world. By 2015, the global photovoltaic systems had an installed capacity of about 140 GW while solar thermal power plants had installed a total installed capacity of 5 GWs ( Hübner et al., 2016) . Photovoltaic systems may be implemented in areas where solar thermal power plants have been deployed. Photovoltaic systems can meet the energy demands from both power plant and residential sectors. However, solar thermal power plants require large-scale projects with capacities of over 20 MW. 

For solar thermal power plants, its initial investments regarding turbines, cost of mirrors, vacuum receivers, efficient heat transfer liquids, lenses and support structures have hindered its rapid development. Most stakeholders will likely look for avenues to reduces the capital investments of solar thermal power plants. If properly implemented, solar thermal power plants may supply about 10% of the global energy needs. However, investment in such projects has remained slow in most countries. However, there has been some probable technological breakthroughs that may enhance the development of solar thermal power plants ( Liu et al., 2016) . For instance, some research teams are evaluating the solar thermal synthesis of certain fuels that are electrochemically consumable or combustible and have adequate energy densities that may be transported for electricity conversion or in various transport applications. The researchers are classified in the “solar fuels” category. It could expand the application of solar thermal power in the energy industry while increasing the market for its technology. Furthermore, the USA has the SunShot Programme, aimed at making solar thermal power technology more competitive. 

Other avenues for development in the solar thermal power plants are distributed combined cycle systems and thermal storage systems. Thermal storage includes storage structures and materials that affect the investment cost and efficiency of solar thermal power systems. Because of costs, temperature limitations, and flammability, thermal oil or steam may not be an attractive option for the high-temperature materials. Although molten salts have been a common alternative in the most recent solar thermal power plants. The most common drawback of molten salts is its decomposition at the high temperatures and its solidification at low temperatures ( Liu et al., 2016) . Therefore, it places a limit on the operating temperature range. Furthermore, research reveals that the temperature range is between 80 0 C and 560 0 C. Therefore, most research will focus on coming up with more efficient heat transfer fluid that has a broader operating temperature range. 

The current concentrated solar power plants are often limited by the material temperature limits that impose certain limits on the effectiveness of the power systems, especially at the turbine inlet temperature. For instance, some of the storage materials utilized in the system may limit the maximum temperature range of their HTF. Deterioration of the molten salts and corrosion increases with temperature increase, and it requires the use of highly expensive storage tank equipment ( Bradshaw, Cordaro, & Siegel, 2009) . These factors reduce the turbine efficiency by placing a limit on the maximum temperature levels. 

The paper analyzes solar thermal power systems, its financial requirements, and its future needs. It explains various solar thermal power plants such as the dish Stirling system, Parabolic Trough Collector Systems, Central Receiver Systems-Power Tower and solar updraft towers. The essay also evaluates the financial needs of the industry that faces various challenges such as the financing cost penalty, the financial investment in the land or solar field are often taxed differently than investments for fossil fuels. It poses various difficulties and the government should formulate policies that will favor investment in solar thermal power systems ( Montes et al., 2009) . It also mentions the future needs and trends of Solar Thermal Power, the countries that are successfully implementing it, and where the future research should focus. The focus should be on technologies that will improve the efficiency of the turbine engine. 

To assist in reaching the environmental and energy goals while promoting green investment, all countries should promulgate regulations and laws that will attract, develop, and encourage investments towards cleaner energy such as solar thermal power systems. Because solar thermal power systems are not yet mature technologies, the industrialized countries should set budgetary allocations and policy support for the research and investment. If countries offer conducive environments, it is highly likely that industrial development in solar thermal power will be greatly improved ( Almsater, Saman, & Bruno, 2016) . The leading research institutions should have incentives and patents for such technologies. Because of the fluctuating fossil fuel price and increasing energy demands for domestic and industrial consumption, the world may be forced to focus on renewable energy especially solar power. However, its capital investment is the main hindrance in its development. There should be measures that will focus on cost reduction and boost its development especially in the developing countries that have plenty of sunlight but limited budgetary allocations. 

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