Solar-hybrid gas turbine power systems offer a high potential for cost reduction of solar power. Such systems were already demonstrated as test systems. For the ...
Solar-Assisted Small Solar Tower Trigeneration Systems R. Buck S. Friedmann German Aerospace Center (DLR), Institute of Technical Thermodynamics, Pfaffenwaldring 38-40, D-70569 Stuttgart, Germany
Solar-hybrid gas turbine power systems offer a high potential for cost reduction of solar power. Such systems were already demonstrated as test systems. For the market introduction of this technology, microturbines in combination with small solar tower plants are a promising option. The combination of a solarized microturbine with an absorption chiller was studied; the results are presented in this paper. The solar-hybrid trigeneration system consists of a small heliostat field, a receiver unit installed on a tower, a modified microturbine, and an absorption chiller. The components are described, as well as the required modifications for integration to the complete system. Several absorption chiller models were reviewed. System configurations were assessed for technical performance and cost. For a representative site, a system layout was made, using selected industrial components. The annual energy yield in power, cooling, and heat was determined. A cost assessment was made to obtain the cost of electricity and cooling power, and eventually additional heat. Various load situations for electric and cooling power were analyzed. The results indicate promising niche applications for the solar-assisted trigeneration of power, heat, and cooling. The potential for improvements in the system configuration and the components is discussed, also the next steps toward market introduction of such systems. 关DOI: 10.1115/1.2769688兴
Introduction The reduction of fossil-fuel based power production by using solar power technology is one important step in the international commitment of CO2 reduction. Electricity from solar radiation can be produced mainly in two ways: directly by using photovoltaic cells and by generating heat to drive a thermodynamic power cycle. Investment cost for photovoltaic systems has come down significantly in the past years, but the electricity production cost is still too high compared to conventional power generation, although significant production quantities have been achieved. Solar thermal power cycles use concentrated solar radiation to heat a medium 共usually liquid or gaseous兲 and transfer the energy to a power cycle for electricity production. Solar-hybrid gas turbine systems offer the potential to convert solar energy with high efficiency, resulting in lower cost for the solar electricity 关1兴. Their advantage, compared to solar-only systems, lies in low additional investment, reduced technical and economical risks due to fully dispatchable power, and higher system efficiency because of reduced part load operation. Another advantage is that until no low-cost storage technologies are available for solar power, a conventional power system has to be kept on standby to compensate the fluctuating power supply of renewable energies. This is in reality another scheme of renewableconventional hybrid power systems but with completely separated system technology leading to economic drawbacks. Real hybrid plants can share much of their system, hence leading to economic advantages. Solar-hybrid gas turbine systems were already demonstrated as test units at a power level of 230 kWe 关2兴. System studies predict attractive electricity cost for the solar fraction, as low as about 8 €cent/ kWh 关3兴. Due to the risk with the implementation of new technologies, the first commercial plants are likely to be built at small power levels, for example, as cogeneration plants 关4兴. The paper describes the assessment of a solar-hybrid microturbine system with 100 kWe in cogeneration configuration, using Contributed by the Solar Energy Engineering Division for publication in the JOURSOLAR ENERGY ENGINEERING. Manuscript received July 25, 2006; final manuscript received March 27, 2007. Review conducted by Abraham Kribus. Paper presented at the 2006 International Solar Energy Coference 共ISEC2006兲, Denver, CO, July 8–13, 2006. NAL OF
the exhaust heat for cooling and heating purposes. Due to the generation of three products 共electric power, cooling power, and heating power兲, such a system is called a trigeneration system. The given information is based on a detailed study on solar-hybrid cogeneration systems for cooling and heating 关5兴.
System Description The solar-hybrid cogeneration system consists of the solar part 共heliostat field and solar receiver兲, a small solarized gas turbine 共microturbine兲 of 100 kWe, and a waste heat recovery system. The latter is composed of an absorption chiller and a heat exchanger for water heating. Figure 1 shows an artist view of such a microtower plant. Figure 4 shows a scheme of the power conversion unit with recuperated microturbine and absorption chiller.
Solar Part The solar part consists of a small heliostat field and a solar receiver. The heliostats are two-axis tracking mirrors, which reflect the sun during the course of the day onto a fixed spot on the top of a tower. In this spot, the entrance aperture of the receiver is located. For the heliostat, dimensions of 5 ⫻ 5.5 m 共27.5 m2兲 were assumed, with a total beam error of 3.55 mrad including sunshape. The specific heliostat cost is 155 € / m2 including installation. The heliostat field layout was done using the HFLCAL optimization code. A short description of this code is given in Ref. 关6兴. The resulting heliostat field, consisting of 19 heliostats, is shown in Fig. 2, with the corresponding annual efficiencies for each heliostat. The position of the tower is represented by the black circle. The receiver aperture is positioned at a height of 29.2 m on the tower, tilted downward by about 43 deg. The design power is 300 kWt and the annual field efficiency reaches 71.9%. From similar field layout cases, it is known that the HFLCAL results are not representing the absolute optimum, but only get close to it. This is caused mainly by two reasons: 共1兲 canting effects are not accurately modeled and 共2兲 limitations for the positioning of the heliostats, which is following specific deployment rules. In this case, a deployment in circles was used, with a bilinear definition of the radius and the angular spacing of the heliostats. This results in four free variables defining the positions of all heliostats. Especially for small fields with few heliostats, this
Fig. 1 Solar microtower plant
leads to acceptable, but not optimal results. In addition, some inaccuracies exist for small heliostat fields, associated with the shading and blocking algorithms in HFLCAL. Additional tools exist to fine tune the layout if required. For this study, no further refinement of the heliostat field was done. For the receiver, a new design with multiple metallic tubes is considered. Multiple tubes are connected in parallel and are arranged in a cavity with an aperture radius of 0.51 m. The annual average efficiency was estimated from HFLCAL as 79.7%. Receiver cost was assumed as 20 k€ 共67 € / kWt兲. Such a receiver is currently under development as part of a new project called SOL-
HYCO 共cofunded by the EC under Contract No. 19830兲. The cost was estimated from material and labor data for a preliminary layout, using six panels with ten parallel tubes each, connected in three serial stages. Although not fully developed yet, the anticipated advantages against pressurized volumetric receivers are lower cost and lower safety requirements. However, the goal of 900° C receiver outlet temperature is quite ambitious with a metallic tube receiver. Depending on the solar system design, the solar share of the solar-hybrid power plant can be close to 100% under design conditions, as the turbine inlet temperature of the microturbine is well below 1000° C. However, much of the total operation time will then be in part load. To achieve longer operating times and to compensate for solar fluctuations, a combustion chamber is foreseen to heat the pressurized air to the required turbine inlet temperature. Thus, the system can reach full availability, independent of the solar irradiation, if required 24 h a day under ideal operating conditions. To ensure stable operation during transient conditions 共e.g., cloud passages兲, a minimum of about 15% of the nominal fuel mass flow is always maintained. This minimum fuel flow is based on discussions with the microturbine manufacturer.
Microturbine The microturbine T100 from TURBEC is a recuperated Brayton cycle with a nominal electrical power of 100 kW at an electrical efficiency of 30% if operated on natural gas. The air mass flow is 0.8 kg/ s at full load; the pressure ratio reaches 4.5. After combustion, the air enters the turbine at a turbine inlet temperature of about 950° C. The package dimensions are 1.81 m共H兲 ⫻ 2.52 m共L兲 ⫻ 0.9 m共W兲; the total weight is 2200 kg.
Fig. 2 Heliostat field layout „colors indicate efficiency…
Table 1 Overview of selected absorption chillers
Cooling capacity 共kW兲 Heat input to generator 共kW兲 COP 共⫺兲 Number of stages Refrigerant/absorbent Specific investment costs 共€ / kWcooling兲
Fig. 3 p , T diagrams of a single effect absorption chiller
For solar operation, the standard microturbine has to be modified as follows: • • • •
integration with a solar receiver 共300 kW thermal power兲 between the pressurized side of the recuperator and the combustion chamber modification of the hardware for emergency procedures adaptation of the control and emergency algorithms modification of the combustion chamber for air inlet temperatures up to 900° C
The value for the electrical power 共100 kW兲 takes into account the internal fuel compressor, which reduces the electrical power output by about 5 kWe. If less fuel is needed 共during solar operating兲, this parasitic power decreases but on the other hand also the fuel mass flow decreases, resulting in less mass flow through the turbine section, i.e., slightly reduced power output. In addition, the solar components introduce additional pressure losses. In consideration of these influences, the solarized microturbine has a lower electrical efficiency than the standard microturbine. The calculations within this paper assume an electrical efficiency for the solarized TURBEC T100 of 28%, independent of the solar fraction.
Absorption Cooling System Absorption chillers are operated between three heat reservoirs and are driven by thermal energy. All state points in the devices of the chiller are dictated by the equilibrium properties of the refrigerant/absorbent mixture. Its function can be described in the p , T diagram 共Clapeyron diagram兲, where the vapor pressures of the refrigerant and the solution are represented by straight lines. Figure 3 describes the process of a single effect absorption chiller. The main difference to a conventional electric chiller is that the refrigerant is compressed by an absorption/desorption process, instead of an electrically driven compressor. The simplest setup is a single effect water/lithium bromide chiller without internal heat recovery. A liquid mixture of refrigerant and absorbent is circulated between absorber and generator with the help of a pump. Heat input to the generator causes the refrigerant to evaporate and the solution becomes more concentrated. The refrigerant which was evaporated at the high pressure level is condensed in the condenser and the liquid is sprayed into the lower pressure evaporator, thereby taking up heat from the water to be cooled. The evaporated refrigerant is finally absorbed in the absorber. Double effect or two stage machines are an approach to improve the efficiency by driving the above described single effect absorption machine by the waste heat of another one that is working at a higher pressure and temperature. If the working fluid of
BROAD BE25
YAZAKI SC30
291 217 1.34 2 water/ lithium bromide 474
105 150 0.7 1 water/ lithium bromide 714
both machines is the same, only an additional condenser-generator pair is needed. The efficiency index of double effect chillers 共coefficient of performance 共COP兲兲 can be near twice that of single effect machines. The COP is expressed as the ratio of the net refrigeration capacity and the driving power. Table 1 shows the main data of two absorption chillers described in this paper. Another commercial absorption chiller from ROBUR was also investigated 关5兴, but did not achieve better performance due to mismatch with the TURBEC T100. The absorption chillers from YAZAKI are well known and successfully used in several applications, also for solar cooling. This hot water driven machine can be adapted easily to the microturbine from TURBEC. Its disadvantage is the relatively low COP, since it is a single effect absorption chiller. The directly exhaust driven chiller from BROAD has a significantly higher COP because of the second stage. As described above, the additional stage requires a higher temperature level of about 160° C, which makes adjustments, in particular, on the part of the microturbine necessary. The microturbine needs a bypass around the recuperator to increase the chiller driving temperature to 415° C, with the corresponding temperature of 170° C of the exhaust stream leaving the absorption chiller. This results in a lower electric efficiency and a temperature decrease of the air to the solar receiver. Therefore, more power needs to be delivered by fuel to achieve the required turbine inlet temperature, as the solar system is not modified for more solar power output. The firing power of the combustion chamber has to be increased from 50 kW for the standard and YAZAKI configuration to 170 kW for operation with the BROAD absorption chiller using partial recuperator bypass. It is clear that this is not an optimal configuration, but the scope of the study was to use existing chiller units without significant modifications. Increasing the power level of the solar part would reduce the firing power when the absorption chiller is operated, but without chiller operation, the solar part would be oversized. Thus, the optimal configuration is strongly dependent on the operation scheme. In Fig. 4, the process scheme for the solarized TURBEC T100 with the BROAD BE25 is shown. The compressed air is preheated in the recuperator before entering the solar receiver. Under design solar power input, the air temperature is increased to 780° C in the receiver. The difference to the required turbine inlet temperature is taken over by the combustion chamber. The turbine exhaust gas flows first through the recuperator to preheat the compressed air. It exits the recuperator at about 415° C when the bypass is partially opened. This remaining heat is used to power the absorption chiller and, as last stage, a heat exchanger for water heating. Figure 5 shows the efficiencies of the different energy conversion modes 共solar/fuel to electricity, solar/fuel to cooling, and solar/fuel to thermal兲 for three system configurations. The first configuration represents the solarized microturbine without absorption chiller 共standard兲. The whole rejected heat is used for service/domestic water heating. The other two configurations utilize the rejected heat from the microturbine for powering the absorption chillers 共YAZAKI/BROAD兲 and the rejected heat from
Table 2 Site data for Seville, Spaina Annual solar direct insolation 共kWh/ m2 A兲 Eectricity tariff 共€cent/kWh兲 Gas tariff 共€cent/kWh兲 Water tariff 共€cent/ m3兲 Waste water tariff 共€cent/ m3兲 a
2000 9.09 2.06 93 51
Tariff status: January 2005.
Table 3 Investment cost Investment cost solarized microturbine 共k€兲 Investment cost solar parts 共k€兲 Investment cost BROAD BE25 共k€兲 Investment cost YAZAKI SC30 共k€兲 Cost for engineering, land, control technology 共k€兲
180 115 138 86 89
Fig. 4 Layout of solarized TURBEC T100 with BROAD BE25
the chillers for water heating. As the different chiller models have different working temperatures, the heat remaining for water heating also differs significantly 共e.g., for the YAZAKI, the inlet to the water heater is 95° C, compared to 170° C for the BROAD兲. Natural gas is assumed as fuel, and the efficiency analysis is based on the lower heating value. No vapor condensation in the exhaust gas heat exchanger is taken into account. Due to the utilization of the solar/fuel energy at three energy levels, the total energy utilization efficiency reaches nearly 100% for the system with the BROAD chiller. This is possible since a large fraction of the excess heat is used in the chiller with a COP higher than 1.3. Losses in the heat and cooling distribution system are not considered. Internal power consumption 共chiller and cooling tower兲 is accounted for by reducing electric power output. The total efficiency can be enhanced by over 20% using the BROAD configuration, relative to the standard configuration. In this configuration, the lower electrical efficiency, which results from bypassing of the recuperator, leads to higher fuel demand to achieve the nominal electric power output of 100 kWe.
Cost Analysis To evaluate the economic performance, the cost for the three configurations is analyzed for the site of Seville 共Spain兲. The utilized site data are given in Table 2. It is assumed that the solarhybrid gas turbine power system is either operated at full load or switched off. Various full load hours are considered for the microturbine, and the operation time of the chiller and water heating system might be lower 共in cases when the microturbine is operated, but no demand for hot water and/or cooling exists兲.
Fig. 5 Composition conditions…
of
the
total
efficiencies
„design
Fig. 6 Required solar electricity payment for cost recovery
The annual cost for the solar-hybrid cogeneration systems is composed mainly of investment, operating, and maintenance costs. The investment costs are shown in Table 3. With an interest rate of 6% and a period of amortization of 20 years, the capital cost for the solarized microturbine amounts to 33 k€ / yr. Operation and maintenance are assumed with a fixed rate of 31 k€ / yr for personnel and consumables, plus the cost for the fuel that depends on operating conditions. Revenues are obtained from selling the generated electricity and the provided cooling and heat. To evaluate the revenues from absorption chillers, the cost for cooling is assumed as the cost of a conventional chiller under the same load situation. Within these calculations, the absorption chiller is operated for up to 4000 h / yr and the microturbine for up to 8000 h / yr. The rejected heat, which cannot be used for operating the absorption chiller, is used for service/domestic water heating. The value for this energy is calculated by the gas tariff and a boiler efficiency of 90% as 2.29 €cent/ kWh. The solar full load operating time2 at the site Seville 共Spain兲 is approximately 1650 h / yr, to be seen in Fig. 6 by the rapidly decreasing curves until this point. If the microturbine is operated for longer time, the costs continue to decrease but less fast. This curve progression points out an important advantage of the hybrid technology. Due to the possibility to operate the hybrid power system 24 h / day, the solar power subsidy required for cost recovery can be significantly reduced. If the rejected heat from the microturbine is used with the BROAD absorption chiller for cooling instead of 2 Annual solar thermal energy at the receiver exit divided by solar design thermal power at the receiver exit.
Fig. 7 Solarized TURBEC T100 and YAZAKI SC30 at 4000 operating h/yr „white dot: profit= −16,200 €…
Fig. 9 Solarized TURBEC T100 and YAZAKI SC30 at 8000 operating h/yr „white dot: profit= + 6000 €…
exclusively heating applications, the costs of the solar cogeneration system can be reduced further. If operated for more than 1850 h / yr, this system is more cost effective than the standard solution with an electric chiller and electricity bought from the grid. This benefit amounts up to 4 €cent/ kWh of solar generated electricity for conditions with more than 4000 h / yr operation. In Figs. 7–10, the annual profits are shown for a solarized microturbine coupled with the absorption chillers from YAZAKI or BROAD, with the gas and electricity tariffs as variables. In addition to the site data for Seville 共Spain兲, it is assumed that the Spanish premium for solar thermal power plants is paid, i.e., the solar fraction of the power production is sold at about 21.62 €cent/ kWh. It should be noted that the current Spanish feed-in law for solar thermal power does not include hybrid options. The four figures show four cases, which differ in the configuration 共BROAD or YAZAKI兲 and in the annual operating time of the microturbine. The absorption chillers are always operated 4000 h / yr. Water heating is used whenever the microturbine is operated. The white dots represent for each case the economic
situation under the tariff conditions given in Table 2 共representative for Spain in January 2005兲. The corresponding profits for these situations are given in brackets in the figure captions. Negative profit means that the revenues are lower than the cost. When the profit is positive, the revenues are higher than the cost. The configuration with the most financially rewarding absorption chiller from BROAD reaches cost recovery at 4500 microturbine operating hours. For this case, the solar share is 29%. Solar share is calculated from the annual solar thermal energy at the receiver exit divided by the total thermal energy input to the microturbine 共fuel+ solar兲. If the microturbine is operated 8000 h / yr, the annual profit sums up to 19,900 €, respectively, 17.9% of the annual costs for the solar driven microturbine at a solar share of 18%. As can be seen for the cases with 8000 h / yr, both systems have positive profits, i.e., it would be cost competitive under the assumed conditions, compared to the conventional solution with electricity from the grid and an electrical chiller system.
Fig. 8 Solarized TURBEC T100 and BROAD BE25 at 4000 operating h/yr „white dot: profit= −2300 €…
Fig. 10 Solarized TURBEC T100 and BROAD BE25 at 8000 operating h/yr „white dot: profit= + 19,900 €…
Conclusions and Recommendations
Fig. 11 Required electricity payment for cost recovery
If a system is operated under different tariff conditions, the economic situation can be estimated from Figs. 7–10 by extracting the profit result for the specific tariff data.
Sensitivity Analysis As reference system for the sensitivity analysis, the configuration with T100 and BROAD BE25 was selected. At an annual operation time of 8000 h, this system reaches cost recovery at an electricity payment of about 9.2 €cent/ kWh both for the solar and the fuel generated electricity. Figure 11 shows the influences on the electricity payment required for cost recovery when varying the cost for heliostats, receiver and O&M. Additionally, the annual solar direct insolation 共DNI兲 is varied. Reducing the cost for O&M leads to the most significant cost reduction for the solarhybrid trigeneration system.
Market Perspective The proposed system will be suited for locations with good solar irradiation levels and a demand both for electricity and cooling and/or heating. At the given power level, this is especially true for tourism installations 共e.g., larger hotel complexes兲. In this case also a demand for water heating 共service water and swimming pool兲 is expected. In addition, a solar-supported 共“green”兲 power system will result in a marketing benefit for those installations. As tourism installations are often located in good solar regions, this can be a significant market potential in the future. As a next step, a closer analysis of this market is planned to verify the potential.
A solar-hybrid microturbine cogeneration system was studied for its thermal performance and economic data. The results indicate that such a system can reach breakeven in certain cases. As reference, conventional supply of power and heat was assumed. The double-effect absorption chiller shows better results both with respect to thermal performance and cost. In cases with longer operation of the hybrid system, the cost situation improves. To take advantage of the high exhaust temperature level of the microturbine, a double-effect absorption chiller should be used, resulting in a high COP. The heat input should be directly from the microturbine exhaust to reduce the number of additional components. The available double-effect absorption chillers do not perfectly match the microturbine conditions. A model with lower power level will better match the exhaust stream conditions of the selected microturbine. This would allow highly efficient cooling production while maintaining the high efficiency of the microturbine without bypassing the recuperator. A prototype of a solar-hybrid microturbine unit will be built in the current SOLHYCO project, cofunded by the EC. This project started in 2006 and the solar-hybrid microturbine system will be commissioned in 2008. This system will be close to a commercial system and can be easily extended with an absorption chiller to a configuration as described in the study.
Acknowledgment This work was funded by the German Federal Environment Ministry 共BMU兲 as part of the CO-MINIT project 共Contract No. 16UM0017兲.
References 关1兴 Kribus, A., Zaibel, R., Carey, D., Segal, A., and Karni, J., 1998, “A SolarDriven Combined Cycle Power Plant,” Sol. Energy, 62共2兲, pp. 121–129. 关2兴 Heller, P., Pfänder, M., Denk, T., Tellez, F., Valverde, A., Fernandez, J., and Ring, A., 2006, “Test and Evaluation of a Solar Powered Gas Turbine System,” Sol. Energy, 80共10兲, pp. 1225–1230. 关3兴 Romero, M., Buck, R., Pacheco, J. E., 2002, “An Update on Solar Central Receiver Systems Projects and Technologies,” J. Sol. Energy Eng., 124, pp. 98–108. 关4兴 Romero, M., Marcos, M. J., Téllez, F., Blanco, M., Fernández, V., Baonza, F., and Berger, S., 1999, “Distributed Power From Solar Tower Systems: A MIUS Approach,” Sol. Energy, 67共4–6兲, pp. 249–264. 关5兴 Friedmann, S., and Buck, R., 2005, “Potenzialanalyse Solarthermische KraftWärme-Kälte-Kopplung 共KWKK兲,” Report of CO-MINIT Project. 关6兴 Schmitz, M., Schwarzbözl, P., Buck, R., and Pitz-Paal, R., 2006, “Assessment of the Potential Improvement Due to Multiple Apertures in Central Receiver Systems With Secondary Concentrators,” Sol. Energy, 80共1兲, pp. 111–120.
