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Energy Procedia 69 (2015) 1241 - 1248
International Conference on Concentrating Solar Power and Chemical Energy Systems,
SolarPACES 2014
STE plants: Beyond dispatchability Firmness of supply and integration with VRE
L. Crespoa
aPresident of the European Solar Thermal Electricity Association (ESTELA), Rue de l'Industrie 10, B-1000 Brussels, Belgium - Tel: +32 (0) 289 325 96 - E-mail: lcr@estelasolar.eu - www.estelasolar.eu
Abstract
The Solar Thermal Electricity (STE) community is certainly concerned about the difference in the deployment trend when compared with Variable Renewable Energies (VRE). The installed capacities in wind and PV have reached 350 GW and 160 GW respectively while STE plants have only achieved 4 GW. The explanation is easy: the current generation costs of wind and PV plants are lower than those of STE plants and a large majority of installations were sited in industrialised countries - where no additional backup was required. Large shares of VRE bring significant reductions on the operation time of flexible conventional plants and even lead to curtailments on renewable energy sources as well.
In developing countries, the situation is quite different. These must not only increase, but mainly multiply their installed capacity in the next years for meeting the demand needs at all times. Therefore, this increase of installed capacity power cannot be exclusively based, e.g. in PV and/or wind plants, as the evening peak has to be covered every day. Such investments in VRE must then be backed up by new combined cycles. This also why STE could already be selected as the best choice in several Sun Belt countries.
However, the main challenge for STE plants is not only to offer some flexible dispatch features, but also to be able to respond to the requested dispatch profile under any circumstances. Achieving this goal with huge solar plans and storage systems will be certainly not cost competitive in the short term. Therefore, "smart" combinations between storage and hybridisation solutions will need to be developed.
In this paper, different conceptual alternatives for firm supply features of STE plants are analysed and the advantages of highperformance hybrid solutions combined with storage capabilities are featured. Nevertheless, a fruitful coexistence and the reaping of synergies with VRE at large regional and seasonal scales are also addressed.
Although there is still a large potential for further VRE plants deployment, policy makers everywhere will realise sooner rather than later, that the true market value of firm supply as delivered by the last generation of STE plants -besides their substantial positive macroeconomic impacts in the countries' economy. And so will hybrid STE plants with storage be the prevailing concept in the future.
© 2015Publishedby ElsevierLtd. Thisisanopenaccess article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).
Peer review by the scientific conference committee of SolarPACES 2014 under responsibility of PSE AG
1876-6102 © 2015 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer review by the scientific conference committee of SolarPACES 2014 under responsibility of PSE AG doi:10.1016/j.egypro.2015.03.161
Keywords: Firmness; Dispatchability; STE hybrid plants; Storage; STE and wind integration
1. Introduction
The share of renewable electricity generation is increasing worldwide. In the last years the investment in renewable power plants has topped the investments in conventional plants but the breakdown into the different renewable technologies has not been even. Wind and PV penetration started to grow at the beginning of the century due to important commitments of industrialised countries to necessary public supports while the first commercial size STE power plant could only be built in 2007 in Europe. The exponential growth of wind parks and PV (domestic installations and large power plants) which should have reached around 350 GW [1] and 160 GW [2] respectively by middle of 2014, along with the time delay for the deployment of STE plants, explained most of the current differences in the cost of electricity production.
However, apart from the virtuous circle "volume - cost reduction", there is a main reason to understand why Variable Renewable Electricity (VRE), like wind and PV, had a quicker deployment: the vast majority of these installations were sited in industrialised countries where no additional backup was required. So it was easy and relatively cheap to improve the "green share" in the generation mix in these countries.
Nevertheless, large shares of VRE do have substantial negative impacts on the operation hours of flexible conventional plants, such as combined cycles, and even lead to curtailments on all renewables. To illustrate this important effect, Table 1 shows the restrictions on the operation of practically all STE plants in Spain on some windy days in March and April 2014.
Table 1. Restrictions on the operation of STE plants in Spain caused by excess of wind
Day Restriction MWh lost
10 March, 2014 No operation until 20:00 1563
18 March, 2014 Operation at minimum technical level 14:00 - 18:40 3246
24 March, 2014 Operation at minimum technical level 10:20 - 19:10 1275
28 March, 2014 Operation at minimum technical level 11:30 - 19:20 1324
30 March, 2014 No operation during the whole day 2319
2 April, 2014 No operation until 20:00 2340
Restrictions set on installing new STE plants due to non-manageable overproduction of wind power combined with non-dispatchable PV generation and the need of releasing high amounts of accumulated water in dams. As a result, all these restrictions represented a negative impact close to 2% in the potential production of STE plants in Spain. This leads to the following considerations:
- The growth of VRE is certainly becoming a serious issue when a certain level of VRE penetration is reached and when discussing how the capacity payments are to be shared.
- The value of STE plants in industrialized countries will become more apparent when worn-out, polluting and/or or risky coal and nuclear plants will be phased out and the need for setting up replacement backup capacities for VRE will need to be decided.
- In developing countries the situation is quite different. These countries need to boost their installed power in the next years in order to respond to the demand needs in all timeframes, i.e. especially around 22:00h, every day. Therefore, the increase of their generation parks cannot be based only on PV/wind plants exclusively. Investments in VRE must be always be backed up by additional investments in combined cycles and in this way, then STE appears to be the best choice in Sun Belt countries. Most of the Feed-in-Tariff granted so far as well as other kinds of support programs didn't differentiate the premium with respect to the time of the day. Lately, the South African program did so, which was perceived as an efficient market signal.
STE plants could, in principle, be designed to respond to a required demand profile by means of large solar multiples in the solar field and oversized storage tanks. But this is certainly not the most cost effective approach. Firmness of supply (or an optimized integration with VRE power plants) can be better achieved by a wise use of flexible fuels, such as biogas or natural gas. This is what is going to be covered in this paper further below.
2. Firmness of supply: STE hybrid plants
2.1 Auxiliary heaters in parallel to the solar field
STE plants can be easily designed to integrate a heater (fed either with a fossil fuel or biomass) which will substitute the source of solar energy when the sun is not shining. The gas heater will be installed in parallel to the solar field and it will heat the same heat transfer fluid that is used in the collectors. Thus, with the same steam generator and turbine, the plant could provide a firm response to the demand requirements, provided that the heater has the thermal power which is required by the turbine to work at nominal conditions. Then, the conversion efficiency when burning the auxiliary fuel will be limited to the nominal efficiency value of the power block while this auxiliary fuel could be burned at much efficiency levels in conventional power plants.
Such systems have in fact been incorporated first in the SEGS plants in California and more recently in practically all Spanish plants, although the thermal power of the heater in the Spanish case was only able to provide around 30% of the required amount for the nominal electrical output of the plant and the production from gas was limited to 15% on yearly basis.
This solution helped for quicker startups of the plants and enhanced the output conditions of the steam generator when there was not enough solar radiation. Nevertheless this solution with such limited power of the heater is not appropriate for firm supply of electricity because the nominal conversion from gas at partial load will not be too high. In the Shams 1 plant, for example, the power output of the gas heater is much higher and the solution makes more sense although the conversion efficiency from gas to electricity will still be much lower than the corresponding one in a combined cycle power plant.
There is also an example for a complete hybrid solar-biomass plant in Spain where the nominal electrical output can be generated either from the sun or from the heat transfer fluid (HTF) biomass heater. This plant can easily be operated in base load but, again, the temperature limitation of the solar HTF limits the conversion efficiency when burning biomass, compared to a specific biomass power plant.
So, offering firm and reliable power supply with these types of properly designed "parallel" hybrid plants is possible but it leads to serious efficiency concerns regarding the conversion into electricity of the gas burnt and hence, this does not seem to be a prevailing option in the future.
Fig. 1. Borges Blanques hybrid solar-biomass power plant, 22.5 MW, Lleida, Spain
2.2 Integrated Solar Combined Cycles
Another proven way of combining solar energy with natural gas is the so called "Integrated Solar Combined Cycle" (ISCC), where a solar field is built next to a combined cycle (CC) plant and the steam produced from solar energy is fed into the conventional plant. The solar field will act as a kind of "gas saver". The equivalent conversion efficiency will be higher than the one corresponding to a STE plant. The cost will be much lower as the collected solar energy will be transformed into electricity through an existing power block.
There are no published results on performance of any of the 4 operational ISCCs in the world - Florida, Morocco, Algeria and Egypt - but there are some concerns about whether the injection of steam coming from a solar plant could compromise, to some extent, the performance of the conventional plant. The design of the combined cycle has an optimum equilibrium between the exhaust energy of the gas turbine and the steam cycle including all the regenerative bleeds. Therefore, coping with the variable production of steam, which will follow the solar radiation curve, requires a more complex control and the overall efficiency of the plant could be also slightly affected. That is why the maximum expected contribution of the solar part with the ISCC approach will seldom exceed 10-15 % per year.
Although it does not seem to be the main trend for the deployment of STE systems in the future, Fresnel collectors with direct steam generation could provide rather competitive solutions with low investment costs in the solar field for this kind of fuel saver concept provided that the cost of the solar thermal collected energy will result cheaper than with parabolic trough systems.
2.3 Solar boosters for coal power plants
Regarding existing coal fired power plants, there is also a possibility of substituting a part of the regenerative bleeds with steam coming from a solar field, increasing the power output of the plant or saving coal for the same production. The easiest way for solar field integration will be consisting in suppressing some bleeds of the turbine to warm the feed-in water. Such concepts are usually known as "solar booster" and the only reference up to now is the 44 MW Kogan Creek coal plant in Australia, which uses Linear Fresnel reflectors in the solar field. The maximum expected contribution and the control issues commented when describing the ISCCs apply also here, although the later ones can be easier to overcome if the substituted bleeds are on the low pressure side of the steam cycle since the maximum solar contribution remains limited when replacing bleed steam.
2.4 Gas and storage: the perfect combination of gas turbines with molten salt STE tower plants
Up to now, hybridization has been approached via installing something in parallel to the solar field (matching the performance constraints of the STE plant) or integrating solar fields into existing high-performance conventional plants with important limitations in the maximum solar share (solving some control issues which might impact the operation of the conventional plant).
The advanced air receiver tower plant, where the gas burner can be mounted in line with the receiver outlet, would use the natural gas at its maximum potential. This concept would be a new kind of ISCC where the solar energy feeds a large part of the energy which is required in the Brayton cycle and the exhaust energy is recovered in a bottomed Rankine cycle. The SOLUGAS project in Seville is an operational pilot project with this idea. Conversion efficiency will be high, but it will not have easy storage possibilities. Thus, it would have to compete with conventional combined cycles on the one side and with non-dispatchable PV plants on the other side. Besides, having a solar receiver and a gas combustion chamber strongly coupled will add control issues and it will result in inflexible operational modes. Air receiver concepts must find decoupling and storing solutions if they want to have a chance in the future.
Yet, a proper integration of gas - natural or biogas - with storage in STE plants can provide the right approach to offer firmness of supply - an important step beyond dispatchability - with high efficiency figures and complete flexibility in the generation of electricity.
Various designs could be feasible along this main idea. Figure 2 presents just one possible solution using a simple gas turbine with a molten salt heat exchanger to recover the exhaust thermal energy of the air turbine. The temperature ranges of the exhaust air fit quite well with the current outlet operational temperatures of the molten salt receivers in tower plants, which are in the range of 560 °C. Therefore the storage system can be charged either when the sun is shining or when the gas turbine is operating at the same conditions. The generation of electricity from the steam cycle is completely decoupled either from the gas turbine or from the solar part. A first reference on this concept has been announced in the IEA STE roadmap [3].
This system has an efficiency figure similar to CCs in the conversion of the thermal energy from the fuel (oil, gas or gasified biomass). It allows for a more flexible and wider variety of operational modes and dispatching profiles. This solution can be offered either for base load operation, with a solar share close to 50%, or to supply firm electricity production at a given time frame every day, with solar shares close to 80%. This is only a question of
Fig. 2. Molten salt receiver solar tower plant and a gas turbine with a molten salt heat recovery heat exchanger (The two generation plants are shown separately for easy understanding of their independence and flexible operation)
proper combination of heliostat field and storage sizes with the rated power of both turbines, which could be the same or not. Normally, the production will come from one of the turbines, but the system can respond to peak demands operating both turbines at the same time. The optimisation of the design will depend on the demand needs and the hourly electricity prices. The additional investment in the solar plant would not be too high as the gas turbines and the heat recovery equipment are commercially available at affordable prices.
The HYSOL www.hysolproject.eu is a pioneering project following this idea, though the demonstration tests will be done in a parabolic trough plant in Spain. As the two turbines are decoupled from the solar field and between themselves - thanks to the two molten salt tank storage systems -, there are no basic concerns about the reliability of this concept. A first analysis of the performances of parabolic trough and central receiver solar power plants has been carried out by J. Servert et all [4]
3. Integration of STE with VRE
As mentioned in the introduction, non-dispatchable VRE technologies, such as wind and PV, have been largely deployed in countries, like Germany, Italy, Spain, and the USA etc., without paying too much attention to their impacts on the electrical systems. The existing backup capacity as well as the grid stability achieved so far in these countries allowed for a quick increase of the share of renewables in their respective electrical systems, although first effects on curtailments start to become visible.
In most of the countries where STE is being deployed, the issue of cost competition among the different technologies was not properly addressed until now. Yet there is a true risk that the price becomes the sole factor for investment decision as VRE are close to cost breakeven with conventional technologies under specific circumstances in various countries. Planners of electrical systems and policy makers must therefore be informed about the necessary dispatch capability across a large part of any generation park considered. Therefore, too simplistic cost-based bidding processes must be complemented with other considerations at system level.
STE - along with hydro and biomass - could play the role of backup for VRE, especially if the interconnections among the countries are reinforced. This would prevent from having to install a large amount of combined cycles as backup in all the countries considered.
As a good example, reinforcing the interconnections among countries with High Voltage Direct Current (HVDC) lines would result in Europe reaping substantial benefits from the monthly and seasonal complementary capacity factors between the wind resources in the Nord See and the solar energy from the Mediterranean countries. In figure 3, one can see how good these two renewable sources complement each other with a true capability to respond to demand needs of many continental countries in Europe.
1 2 3 4 5 S 7 8 9 10 11 12 Months 123456789 10 li 12
Fig. 3. Capacity factors of offshore wind parks in the Northern See (on the left) and solar plants in Southern Europe (on the right) Source: Own elaboration from several operational references
Taken together in a proper weighted way, a significant part of the electricity demand in Germany could be supplied by the combined production from wind, PV and STE plants with a reasonable generation park as shown in figure 4.
50 40 | 30 20
Fig. 4. Monthly electricity demand in Germany and potential combined supply from Offshore wind, PV and STE Source: IEA and own elaboration from operational references in Spain
The electricity production has been taken from the current accumulated levels of the operational STE and PV plants in Spain.
Even at local level good examples can be found of places where wind parks and STE plants are connected to the same distribution substation. In the province of Granada, Spain, near the three Andasol STE plants, large wind parks are installed achieving together a high capacity factor throughout the year at the central substation of Hueneja, as commented by the operational teams of the Andasol plants.
Fig. 5 Andasol plants (150 MW) in the front and wind parks (200 MW) at the rear in the province of Granada, Spain
Joint STE and PV can be also a good solution for matching the demand requirements. The PV panels could provide the electricity during sunny hours while the STE part could mainly provide the requested power to cover the evening peaks and/or the early morning requirements thanks to its storage capabilities. Depending on the dispatch profile requirements and the cost of electricity at different times of the day throughout the year, the installation of a gas turbine could also be considered.
Fig. 6. PS10, 10 MW STE plant at the rear and a two-axis tracking low concentration 1 MW PV plant at the front
in the Solucar complex, Seville, Spain
The PS10 has been connecting to the grid since 2007 and it was the first commercial plant of the new STE era after the "long and dark solar thermal electricity night" since the last SEGS plant was commissioned in 1991. The picture of the PS10 has an important symbolic value, but it could well turn out that the coincidence of the two plants - STE and PV although they are not operationally integrated - hints at the future of solar power in Europe.
4. Conclusions
The deployment of renewable energy power plants took place very quickly over the last few years. In fact, the investments in renewable power have been higher than in conventional plants but the dispatchability requirements have not been duly considered until now. The result was an exponential growth of non-dispatchable and cheaper technologies with an impressive impact on their cost reduction curves. This process has taken place above all in industrialized countries, but it is not sustainable in fast emerging countries where fulfilling the demand needs at any time is a must.
STE offers big advantages in terms of quality of supply and macroeconomic impacts on countries' economies and these advantages will certainly be better valuated by both policy makers and planners.
However, the new STE projects must go one step forward offering not only dispatchability, but also firmness of supply. Combination of gas turbines with STE plants with storage systems - e. g. molten salt towers - will position the STE technologies as an unbeatable choice.
This stands also for Europe, where the targeted generation mix in 2030 and beyond is seen as carbon free: for this reason, the role of STE plants will be essential. Nevertheless, important synergies with other VRE will be found taking into account the seasonal complementarity with offshore winds at wide regional level, provided that sufficient interconnection capacity is created and available, and perhaps also with PV in joint plant concepts.
References
[1] GWEC reported a Wind capacity installed of 318 GW by the end of 2013 in its document Global installed wind power capacity in 2013.
[2] EPIA reflects in its Global Market Outlook a capacity installed of 139 GW by the end of 2013.
[3] Technology Roadmap, Solar Thermal Electricity, International Energy Agency, Paris, 2014
[4] Base Case Analysis of a HYSOL Power Plant, J Servert el all, Proceedings, SolarPACES Conference 2014
