2. LITERATURE REVIEW
2.2 Parabolic trough systems
2.2.5 Power plants
Three types of power plants or cycles are commonly used for parabolic trough power plants:
the steam Rankine cycle, the organic Rankine cycle (ORC) and the combined cycle (which also utilizes a Rankine cycle). There are also differing techniques to integrate the thermal energy generated in the solar field into the power cycle.
The thermal-to-electric energy converter most often used for parabolic trough systems is a turbine. A steam or organic working fluid turbine uses a Rankine cycle to convert heat energy into rotational energy which is then converted to electrical energy via the windings on the shaft (rotor) and the body (stator) of the turbine alternator.
Cumulative temperature measurements by the SAWS report that Upington has a mean temperature of 35°C. Considering that the HTF heats the water used in the Rankine to around 393°C, a theoretical maximum efficiency for any Rankine cycle operating at this temperature can be calculated using the Carnot efficiency ( ) as follows:
1 1 308
666 0.54 54%
(2.4)
where and are the cold reservoir temperature at ambient and boiler temperature in Kelvin respectively. Adding the parasitic system losses incurred by the major components of a Rankine cycle, namely the pumps, boiler, turbine and condenser will reduce this value significantly. Plants using conventional steam Rankine cycles report efficiencies in the order of 38% (Kelly, 2006a). Also evident from the equation is that the higher the temperature in a Rankine cycle the more efficient the energy conversion. Hence, HTF temperature is a critical contributor to the efficiency of a pure Rankine cycle.
The overall efficiency of a Rankine cycle is characterized mainly by the difference in the temperature and pressure of steam entering a turbine to that of steam exiting it. One of the central design considerations in Rankine cycle CSP applications is the heat rejection system.
The two commonly used methods are dry and wet cooling. Wet cooling, which provides a lower condensation temperature, improves the expansion ratio and ultimately the electrical energy derived from work performed by the steam. However, this comes at the cost of using considerable amounts of water. Considering that Upington and its surrounds are arid, this poses an environmental concern. CSP plants utilizing dry cooling are estimated to produce between four and nine percent less electricity annually than plants employing wet cooling (Kelly, 2006b). However, Kelly (2006b) also estimates that dry cooling uses only eight percent of the water required by wet cooling in equivalent CSP plants.
Plant operator experience (Scott and Lee, 2006) suggests that wet cooling is more cost- effective, has lower operation and maintenance (O&M) costs, and provides higher power- cycle efficiencies particularly during the hotter summer months than dry cooling. This along with the higher capital costs of dry-cooling systems requires CSP plant designers to chose
between obtaining higher solar-to-electrical efficiency at lower capital cost, or losing solar-to- electrical efficiency and increasing capital spend to protect water resources in the area.
As parabolic trough- and conventional power plants utilize the same steam turbine technology (with only the steam-heating technology differing), they are easily hybridized. Thus a coal- fired boiler could easily supplement a parabolic trough plant during cloudy periods or overnight if firm base-load power generation is required. However, the startup times of coal- fired boilers often lead to the use of natural gas standby turbines so that the nameplate capacity of the plant can be maintained during peak load times.
2.2.5.1 Steam rankine cycle
All of the SEGS plants and a considerable number of new plant projects entering their Engineering, Procurement and Construction (EPC) phases use steam Rankine cycles. As of 2008 Siemens had secured orders for forty-five of their SST range of steam turbines for CSP plants worldwide (Siemens, 2008).
CSP plants may utilize either high-, intermediate- or low-pressure steam turbines. Kelly (2006a) suggests that intermediate- or low-pressure turbines between 50 MWe and 140 MWe
provide higher expansion efficiencies than similarly sized high-pressure units. And high- pressure turbines between 140 MWe and 220 MWe provide higher expansion efficiencies than similarly sized intermediate- or low-pressure units. Non-uniform aerodynamic loads at the end of the turbine blades from fixed steam leakage in the high-pressure turbine represented a smaller proportion of the total flow past the blades as blade-length was increased. Hence high-pressure turbines are better suited to large CSP plants and intermediate- or low-pressure turbines are more efficient energy converters in intermediate-sized CSP plants.
According to Feldhoff et al. (2009) the principal difference between a steam turbine used for a CSP plant, and a steam turbine used for a conventional fossil fired power plant is the cycling rate of start-ups and shut downs. A conventional plant turbine may be shut down several times a year whereas a CSP unit is started up and shut down daily. This daily cycling requires careful monitoring of the main steam temperature and pressure of the turbine.
Steam Rankine cycles are made more efficient by using waste heat to increase the temperature of the steam in the boiler and hence increase the Carnot efficiency. This is commonly termed as reheat technology and most turbines used in CSP applications are reheat steam turbines.
The power cycle model in SAM for parabolic trough plants is a Rankine cycle model. It uses inputs to characterise the power plant’s capacity, cycle conversion efficiency, control philosophy and cooling system. It allows for different modes of fossil fuel backup including minimum backup level or supplemental operation. Start up and standby times can also be defined.
2.2.5.2 Organic rankine cycle
In the Organic Rankine Cycle (ORC) an organic chemical is heated to form a gas which drives a turbine. Using refrigerants (e.g. freon, butane, ammonia) which boil at extremely low temperatures of typically 66°C, generating significant pressures, the ORC can occur at much lower temperatures than the steam Rankine cycle. In an ORC an evaporator replaces the boiler and condenser only requires ambient air to cool the gas leaving the turbine into a liquid.
Prabhu (2006) performed a study on the Solar Trough Organic Rankine Cycle Electricity System (STORES) concept and found that even though the steam Rankine cycle has thermal efficiencies of between fifteen and twenty-five percent higher than the ORC; the ORC has other benefits that make it a viable alternative in 1 MW to 10 MW plant sizes. It is more efficient when the ambient temperature is low (this matches the load profile of South African communities which consume more electricity during winter months) and can be operated with minimally skilled operators and maintenance crew which makes it potentially attractive for Upington.
In order to enhance the efficiency of ORC turbines, two ORCs are cascaded with the first higher temperature ORC loop utilising a refrigerant with a higher boiling point and the second lower temperature ORC loop using waste heat from the first loop to evaporate a second refrigerant with a lower boiling point.
Two methods normally used to increase the thermal efficiency of a cascaded ORC are recuperation and selecting an HTF that can operate at a higher temperature. Using the temperature of the gas exiting the turbine’s exhaust to preheat the chemical fluid before it is vapourised is termed recuperation. By allowing the HTF temperature to be as high as possible, and hence increasing the temperature at which the ORC occurs, a higher thermal efficiency can be attained.
Prabhu, (2006) states that “the recuperated cascade cycle is the most thermally efficient”
ORC. But he also admits that even the efficient ORC systems “are less efficient than extraction steam cycles”. They still pose a promising alternative for the Upington area.
2.2.5.3 Combined cycle
Integrating a CSP plant with a conventional coal-fired steam or gas-fired power plant can be an efficient way to combine the base-load reliability of conventional boilers or gas-turbines with the midday peak-load tracking characteristic of a CSP plant provided it has been started up in the morning and is past its thermal inertia.
The steam turbine of a coal fired power plant may be oversized to provide either a gas- or solar-powered supplement to the boiler’s capacity and hence run the main turbine at full capacity. During the day and early night the turbine could be supplemented with solar power and stored solar energy respectively. During the later hours of the night the electricity demand may reduce to a level where the turbine need only run at the capacity of the boiler until the next morning. If a morning peak in demand is required before sunrise, the gas turbine could augment the steam supply to the main turbine.