At the short and medium terms, concentrating solar power (CSP) will share the scenario with conventional thermal power plants. In such a context, integrated solar combined cycles (ISCC) are an interesting choice for power generation because hybridisation provides a good use of fossil and solar resources, obtaining higher efficiency than using solar dedicated and conventional combined cycles separately.
Solar combined cycles have been studied since the late 90 s. The concept was initially proposed by Luz Solar International (Johansson et al. [1
]). Early studies were based on the PTC technology of SEGS (Solar Energy Generating Systems) plants installed in California [2
]. At the beginning of the century, the research in the field spread out as a result of the installation of some plants thanks to subsidies provided by the agency Global Environment Facility to countries such as Egypt, Morocco, India and Mexico [5
]. These studies deal mainly with the economic feasibility of the different choices and the production. In recent years, interest in these plants has grown considerably, as it is concluded in [9
], and novel forms of integration [11
] and optimization [12
] are common in the technical literature.
To date, there are only a small number of solar plants based on a combined cycle. Some of them are Ain Beni Mathar (Morocco), Hassi R’Mel (Algeria), Kuraymat (Egypt), Martin Next Generation Solar Energy Center (USA), Agua Prieta II (Mexico), Archimede (Italy) and Yazd (Iran). There are others planned, such as Ningxia (China), Palmdale Hybrid Power Plant and Victorville 2 Hybrid Power Plant (USA), Abdaliya (Kuwait) or Duba 1 ISCC (Saudi Arabia) [9
]. Many of the above plants include multiple gas turbines and HRSGs and a steam turbine [9
configurations) and a solar field in parallel to the heat recovery steam generators (HRSG). Solar energy is collected using PTC and it is transferred to the steam cycle using an intermediate heat transfer fluid (HTF). HTF is usually thermal oil except in Archimede power plant that uses molten salts [5
]. Besides, solar energy is used to evaporate the water of the high pressure level (for example, Hassi R’Mel [5
] and Yazd [6
]), although some plants include a slight degree of solar steam superheating (Ain Beni Mathar [9
], Kuraymat [9
]), water preheating (Victorville 2 [18
]) or even preheating and superheating (Archimede [8
Thus, it may be considered that the state-of-the-art in solar combined cycles consists of a conventional combined cycle in which solar energy is integrated into the steam cycle, usually at the high pressure level. As it is known, such a hybridisation provides some synergies during the yearly operation. The main reason for those synergies is that the most demanding conditions for combined cycle gas turbines (CCGT) (high ambient temperatures) correlate well with the optimal conditions for CSP, which favour the integrated behaviour and efficiency of the ISCC [19
]. The purpose of ISCC technology is either to increase of the power plant or, alternatively, to reduce the consumption of fossil fuel. This allows the use of solar resource while advancing in the learning curves of solar technology, with moderate investment and risk due to the hybridization.
As commented above, the use of thermal oil as HTF is the most conventional choice, although some other alternatives have been proposed, for example, molten salts [8
], direct steam generation (DSG) [22
], and CO2
Studies like [25
] show the prevalence of PTC in the field of ISCC. However, like in the case of pure CSP, there are other choices to integrate solar energy, especially CT [26
] and LFR [27
]. The reasons to introduce these technologies are the same as in solar power plants: CT is considered as the technology with highest potential to reach high temperatures at industrial power rates and LFR, despite its late development compared to PTC, has some potential to reduce the levelized cost of energy.
In addition to the conventional ISCC (solar integration into the steam cycle), other possibilities have been proposed. For example, in Reference [28
] solar energy is integrated into the gas turbine, preheating the air coming from the compressor before it is introduced in the combustion chamber. In this case, the technology used is again PTC. Other options are either the use of CT in parallel with the exhaust gas of a gas turbine [29
], or the use of CT instead of PTC to preheat the air exiting from the compressor [30
], similarly to [28
]. All these configurations are less developed than the conventional ISCC.
In order to cover all these possibilities, the present paper compares the annual performance of fuel and solar hybrid combined cycles using these three different solar concentration technologies—PTC, LFR and CT—and each technology is proposed to integrate solar energy into the combined cycle in two different ways. The first one is based on the use solar energy for evaporating water of the steam cycle by means of DSG. Maximum working pressure and temperature are about 100 bar and 310 °C, respectively. The other one is based on the use solar energy to preheat the pressurized air at the exit of the compressor of the gas turbine before it is introduced in the combustion chamber, reducing the fuel consumption required for a given turbine inlet temperature. In these cases, maximum considered pressure and temperature are 20 bar and 500 °C (which is an optimistic value for LFR and even PTC technologies). The comparative analysis is made considering the thermal behaviour of the different technologies. Economic aspects are out of the scope of the present work, although some preliminary results are commented.
In the following sections, the proposed configurations are introduced, then the methodology and the figures of merit used for the comparisons are presented and finally the results and the conclusions are shown.
4. Results and Discussion
shows the yearly production of the six configurations analysed and the reference CCGT. As it observed, configurations that introduce solar energy to the steam cycle through DSG increase the yearly production, since the solar energy is added to the fossil fuel resource. Comparing solar technologies, PTC is the best one while LFR presents the worst results. Regarding the location, the reference CCGT presents lower production in Las Vegas but, due to the hard climatology, ISCC configurations improve the results and become better than in Almeria.
On the contrary, when the solar contribution is integrated into the gas turbine, the yearly production is very similar to the reference one because solar energy replaces the saved fossil fuel, although it slightly decreases due to the additional pressure drop introduced by the solar field. All solar technologies behave similarly although, in this case, the best one is CT. The behaviour is similar in both locations and the production is reduced roughly a 0.5% at both sites.
shows the gross solar energy contribution and the saved fuel in terms of energy. In the case of DSG technologies, solar contribution is higher using PTC than using other technologies, and it is also higher in Las Vegas than in Almeria. In the case of solar integration into the gas turbine, gross solar contribution is higher as consequence of the larger solar field. Finally, fuel consumption is not modified in configurations that integrate the solar energy to the steam cycle, and it decreases when solar energy is integrated into the gas turbine.
Regarding the efficiencies, Figure 11
shows that DSG configurations lead to lower global efficiencies because the supplementary heat is integrated into the bottoming cycle, with lower efficiency than the combined cycle. Despite that fact, the global efficiency is high and the solar-to-electricity efficiency is higher than the thermal efficiency reached by conventional solar thermal power plants. Besides, Figure 11
also shows that PTC is the most suitable technology for DSG. Finally, due to the hard climatology of Las Vegas, that location presents better efficiency than Almeria.
In configurations with solar integration into the gas turbine, both the global efficiency and the solar-to-electricity efficiencies are higher than those obtained with DSG. The global efficiency is quite similar to that of the reference CCGT, although it is slightly lower due to the higher pressure drops. The solar-to-electricity efficiencies are significantly higher than that obtained by DSG configurations, except for LFR. Finally, efficiencies are higher in Las Vegas than in Almeria.
Besides, Figure 12
shows the heat rates reached by the different configurations. All configurations improve the performance over the reference CCGT, decreasing the heat rates. In the case of DSG, the best performance is reached by PTC and the best location is Las Vegas, accordingly to the previous results. In the case of air preheating of the gas turbine, heat rate decreases significantly except for LFR. Again, results in Las Vegas are better than in Almeria.
Although the economic assessment is compulsory for promoters and decision makers, an in-depth economic analysis is out of the scope of the paper. The main reason is that thermal behaviour and performance can advance moderately during the years but, nowadays, economic frame in CSP is rather variable (I.e. two years ago, generating cost of solar thermal power plant was above 15 c$/kWh while last year this value decreased to 6 c$/kWh.). In such circumstances, the use of costing models for all the technologies analysed in the paper should introduce high uncertainties that are avoided if the scope is limited to the thermal behaviour.
Nevertheless, in order to provide some information at this regard, Table 4
gives some economic results using a fixed economic scenario typical of several years ago. Taking into account that the generating cost of the reference CCGT results 8.82 c€/kWh in Almeria and 8.84 c€/kWh in Las Vegas (due to the lower annual yield), it is observed that PTC-DSG can be competitive in both sites, while LFR-DSG and CT-DSG improve the economic results in Las Vegas but not in Almeria. Given the proposed frame, economic results are better for solar integration into the gas turbine than for DSG configurations. PTC-GT and CT-GT show interesting results and feasibility of LFR-GT is questionable in Las Vegas not advisable in Almeria.
In this work six hybrid fuel-solar combined cycles have been analysed. Three concentrating solar technologies have been considered: namely parabolic trough collectors, linear Fresnel reflectors and central tower receiver. Each technology was analysed considering two ways of solar integration: direct steam generation in parallel with the high pressure evaporator of the heat recovery steam generator and air preheating at the exit of the compressor of the gas turbine.
The configurations were simulated in two different locations, Almeria and Las Vegas, obtaining as results the yearly production, the heat rate and the global and solar-to-electricity efficiencies. Regarding this last one, a new equation has been proposed.
It is important to point out that the analyses are focussed on the thermal behaviour of the plant without considering transient effects due to the climatology variation (for example, intermittent clouds). However, there should be some improvement potential in central tower configurations, since their designs have not been optimised for the selected locations, and substantial improvement potential in Fresnel ones, as they are the cheapest technology and the solar fields have not been optimised. Regarding the results, the following conclusions are obtained:
Integrated solar combined cycles using direct steam generation improve the yearly production because solar contribution increases the steam generation. Conversely, solar air preheating allows saving fuel instead of increasing production and they reduce slightly the yearly production due to the incremental pressure drop.
The solar-to-electricity efficiency is high in all configurations, particularly using air preheating.
Performance is better in Las Vegas than in Almeria for all configurations, due to the desertic climatology.
In terms of energy performance, parabolic trough is the best technology for direct steam generation. For air preheating, parabolic trough and central towers behave similarly, and linear Fresnel reflector is the worst.