TERMOSOLAR BORGES: A THERMOSOLAR HYBRID PLANT WITH BIOMASS

Transcription

1 TERMOSOLAR BORGES: A THERMOSOLAR HYBRID PLANT WITH BIOMASS A. COT*, A. AMETLLER*, J. VALL-LLOVERA*, J. AGUILÓ* AND J.M. ARQUÉ* * COMSA EMTE MEDIO AMBIENTE, ITG, Av. Roma 25, Barcelona, Spain SUMMARY: Since the publication of the last Spanish RD 661/2007, thermo solar energy plants are considered and several projects are under construction and someone in operation. These Concentrated Solar Power (CSP) plants produce electricity by converting solar energy into high temperature heat using diverse mirror configurations. The heat is then used to produce electricity through a conventional generator system using turbine. The ABANTIA and COMSA EMTE innovative solution: the hybridizaton of the CSP plant with biomass fueled boilers. The Termosolar Borges CSP hybrid plant is an innovative solution for thermo solar energy plants allowing a higher electric production per year and a previewed improvement in the stability of the system. 1. INTRODUCTION With the increasing concern for the environment over the last decades, in particular related to climate change, several initiatives from governments and public institutions have gone some way to improving the uptake of renewable energy sources as a viable alternative to traditional energy sources, such as coal and oil. In addition, energy security concerns are pushing governments of various countries to include renewable sources in their national energy portfolios. Spanish government has taken appropriate initiatives to formulate feed-in tariff laws to promote renewable energy plants. Since the publication of the last Spanish RD 661/2007, thermo solar energy plants are considered and several projects are under construction and someone in operation. These Concentrated Solar Power (CSP) plants produce electricity by converting solar energy into high temperature heat using diverse mirror configurations. The heat is then used to produce electricity through a conventional generator system using turbine. Currently, research is undertaken on various CSP technologies for varying levels of high temperature generation capabilities and the storage of thermal energy to increase the number of operation hours of the CSP plants. Actually, most of the CSP projects under constructions employ the cylindrical-parabolic system. It consists of trough-shaped mirror reflectors to concentrate solar radiation on to receiver tubes containing thermal transfer fluid which is heated to produce steam. The ABANTIA and COMSA EMTE Proceedings Venice 2010, Third International Symposium on Energy from Biomass and Waste Venice, Italy; 8-11 November by CISA, Environmental Sanitary Engineering Centre, Italy

2 innovative solution: the hybridization of CSP plant with biomass fueled boilers. The aim of this solution is to reach a higher electric production, a major stability in the operation of the thermal system and a higher renewable output of the plant. 2. DESCRIPTION OF THE HYBRID PLANT 2.1 Main changes A thermosolar plant is mainly composed by 4 blocks: (1) the solar field; (2) the thermal block; (3) the electrical block and (4) the balance of the plant (BOP, that is, auxiliary systems). Compared to a conventional CSP plant, the block most affected by the development of the hybridization with biomass is the thermal block. Some specific biomass boilers have been added and several circuits and the control system of the CSP plant are entirely modified. The power block will be slightly affected by the emergence of self-supply because of all the shredding and storage of biomass. The thermal block is the intermediate responsible for the transfer of solar heat transfer fluid accumulated in the electrical block by conversion to thermal energy as steam turbines that changes its mechanical energy into electricity. Basically consists of three items of process, which are: (1) hybrid boilers with biomass and gas ventures, (2) the steam generator itself, (3) a small conventional auxiliary boiler. 2.2 Hybridization with biomass and gas boilers The functionality of the hybrid boilers is double; they can use biomass or natural gas as a fuel depending on the meteorological conditions. Furthermore, an own developed control system regulates the stability of the steam input for the turbine. For example, during the day, to cope with fast transient lowering of radiation (mainly due to the passage of clouds), boilers use natural gas as fuel, allowing a faster and more accurate control. SF TB EB BOP SF = Solar Field TB = Thermal Block EB = Electrical Block BOP= Auxiliares Figure 1. Scheme of a thermosolar plant.

3 At night or in completely cloudy days (long periods without radiation) the fuel used by the boilers will be the biomass. The power of all the boilers is 36 MWt in the circuit. The choice of this power for the boiler is motivated by being able to achieve at least 50% of the workload of the turbine running at night. Below this load, the turbine efficiency decreases sharply. The boilers are mainly composed of the following elements: Biomass feeding system Furnace for biomass Set of natural gas burners Heat recovery system Overall, the operation is as follows: the biomass is feeded in the furnace with the aid of a special designed system. The furnace is mounted in a manner fully integrated with the boiler. The combustion air gradually replenishes the different areas of the hearth to ensure complete combustion. Combustion air is preheated by the economizer. To help the boilers in the transitional arrangements, (i.e. burning wetter biomass than usual), natural gas burners become operative for a steam outlet temperature of 375 º C. 2.3 Auxiliary boiler The hybrid CSP plant has another steam boiler for assistance. His steam generation capacity exceeds 3 tons/h at 15 bar (a) to meet particular needs of the facility. Figure 2. Detailed diagram for the Termosolar Borges hybrid CSP plant. Table 1 - Electrical production for each energy source. Energy source Yearly electrical production [MWh/y] Solar Biomass Natural Gas TOTAL

4 3. RESULTS AND DISCUSSION The Termosolar Borges plant has a nominal power production of 24,8 MWel, and the surface occupied rises to square meters. The direct irradiation assumed in the project is kwh/m2 year. The biomass fuels are mainly forest residue and agricultural crops collected from an extended area in Catalonia. Several studies of biomass availability in the area have been done. The biomass input is approximately tones per year at 45% humidity. The Termosolar Borges CSP plant will produce electric energy from the following energy sources: Solar, biomass and natural gas. According the meteorological conditions of the site Borges Blanques (Catalonia, Spain) and the engineering Project of the CSP hybrid plant, the electrical productions are showed in Table 1. The reliability of renewable energy technologies, including concentrated solar power (CSP), has increased whilst the costs have fallen. The economics of a CSP installation is strongly dependent upon its size. The size is defined in terms of the power output, but it is also directly related to land area occupied. The developments in the technology will also ensure that the system costs decrease and cost reductions are expected as a result of a combination of several factors. The costs of a CSP system can, broadly speaking, be split into solar costs and non-solar costs. Reduction in relation to solar costs lies in mass production leading to economies of scale and in the development of innovative solar collectors along with the development of novel optical systems. Non-solar costs will be reduced by the development of simpler and more efficient heat transport schemes, more efficient power cycles and the hybridization with other renewable and conventional energy sources. Figure 3 shows the CSP map in Spain and the unique CSP plant located in the north area, with a lower solar irradiation over the year, is the Termosolar Borges. The hybridization of CSP with biomass allows expanding this technology in northern areas and opens the use of multiple energy sources for the CSP plants. Operative In works Assigned Figure 3. Geographic situation of Termosolar Borges in the NorthEast of Spain.

5 5. CONCLUSIONS Since the publication of the last Spanish RD 661/2007, CSP plants are considered and several projects are under construction and someone in operation. The ABANTIA and COMSA EMTE innovative solution: the hybrid CSP plant with biomass. This solution provides a higher electric production, a major stability in the operation of the thermal system and a higher renewable output of the plant. Moreover, the hybridization of CSP with biomass allows expanding this technology in northern areas and opens the use of multiple energy sources for the CSP plants. REFERENCES Cohen G. E., Kearney D. W. and Kolb G. G. (1999) Final report on the operation and maintenance improvement program for concentrating solar power plants; NREL publications. SAND Sargent & Lundy LLC Consulting Group (2003) Assessment of parabolic trough and power tower solar technology cost and performance forecasts.; NREL publications/sr Price H. and Hassani V. (2002) Modular trough power plant cycle and systems analysis; NREL publications/tp Spanish Royal Decree 661/2007, Regulación de actividad de producción de energía eléctrica en régimen especial.