Development of solar-hybrid receiver components

A solar receiver should have been developed based on an innovative profiled multi layer (PML) tube concept. A PML-tube consists of very resistant outer layer made of heat resistant steel-alloys and an inner layer made of a heat conductive copper layer.



This technology can enhance the heat transfer from the irradiated tube wall to the gas and allow for reduced temperature differences on the circumference of the tube, thus reducing stress and leading to higher life time. During the course of the project, the manufacturing method of such tubes was developed and first samples were manufactured. Test tubes up to a length of 1.3 m were tested in a laboratory setup. It could be shown, that the temperature difference on the tube circumference could be reduced from 73.7 to 14.1 °C (80.9 % reduction). In the current development stage the inter-metallic connection is still vulnerable to temperature cycling.

Due to time delays in the development of the manufacturing method for the solar receiver of the prototype microturbine system it was decided to built it from mono-layer tubes. The receiver has the same design as with PML tubes, such allowing a later replacement of the tubes.

Solar Component Qualification Test

To elaborate the use of bio-diesel for 100% renewable operation of such plant, a study was realized and necessary modifications for a turbine system developed. The operation with biodiesel was then implemented in the former SOLGATE test setup consisting of a 250 kW airborne gas turbine with three solar receivers. During this project the solar hybrid operation had been successfully demonstrated in 2003, but with kerosene as backup fuel. Now this system was modified and equipped for biodiesel operation. It could be shown, that these adjustments were allowing a flawless operation without problems and without significant change of system behaviour. Unfortunately the results from these tests were too late to be implemented in the design of the microturbine.

Figure 2 - Scheme of test setup at CESA-tower

Figure 3 - Test setup

Development of Solar-Hybrid Microturbine Cogeneration Unit, System Test and Evaluation

The design of the Turbec T100 microturbine had to be modified from the original cogeneration unit since the operation conditions as solar hybrid unit are quite different. The solar receiver adds additional volume and acts as heat sink during startup which requires modifications of the control system. The flow paths had to be improved and pressure losses in the system optimized. The combustor had to be modified due to the different cooling necessities stemming from higher temperatures due to the solar receiver.

The solar receiver for the Turbec T100 microturbine was manufactured in spring 2009. The test bed in the CESA-1 tower was prepared and a test plan defined. The receiver cavity was designed, manufactured and assembled. In summer 2009 the complete system could be built up in the test facility. Electrical wiring was completed and the flux measurement system prepared. A data acquisition system had to be installed and programmed. First system tests started in autumn 2009 showing still major initial problems with the turbine operation and control which could not been solved before spring 2010. Then, the system was successfully tested through a period of more than 165 hours of turbine operation whereof 100 hours were solar operation. The system was operated at design conditions of 800°C receiver outlet temperature. Unfortunately the receiver efficiency could not been measured with the desired accuracy, since the receiver cavity showed defects already after a few testing hours. This led to major heat leaks which could not been precisely determined. Nevertheless the complete system behaved very good and solar hybrid operation could be demonstrated successfully. Also the first solar-only gas turbine operation worldwide of two hours of operation was achieved and it could be shown that a receiver window has a positive effect on the system performance, especially for setups with receivers not facing downwards.

Figure 4 - Solhyco system setup

Figure 5 - Solar Operation

Commercial system layout and cost analysis

Different design variations for Cogeneration systems and Combined Cycle Systems were analyzed and the cost for electricity generation evaluated. For the small cogeneration systems (100kWe), the final receiver design resulted in a more expensive layout leading to overall system cost of 3.440 €/kW. The cost study shows, that with this layout the cost for electricity generation for a good solar site in Algeria is 0,101 €/kWh in an operating scheme with 25 % solar share. To be fully competitive, the system must reduce the LEC by only 20% which seems to be feasible. For the large CC systems a 21 MW system was selected and evaluated. The LEC of such system is 0,078 €/kWh being operated in base load configuration with a solar share of 18 %. For solar shares of 35 % the LEC will be 0.094 €/kWh. This plant would be operated in mid load for 4000 hours per year. It could be shown that the solar hybrid CC system can provide dispatchable power at a high conversion efficiency of 48 %.

Market assessment

The Mediterranean market was studied in detail to elaborate the market potential for the Solhyco technology. The market assessment showed that in most countries the market for hybrid systems is not very well prepared since feed-in regulations, if existing, prefer storage against hybrid solutions. However, the Algerian market, providing also a great solar resource, is offering adequate feed-in regulations for hybrid systems and is identified as the preferred actual market in the Mediterranean. This is especially valid for the big CC plants, since the generation cost and such the profitability is advantageous. If the above mentioned CC plant of 21 MW is operated with a 25 % solar share, the LEC is 0.085 €/kWh and thus close to be profitable in the Algerian market.

For the Brazilian market assessment three cases for different cogeneration applications with the solar-hybrid gas turbine were studied. The analysis of solar radiation potential in Brazil was finished and the data provided for the case study. Dissemination activities were accomplished on the Brazilian national solar congress and on the conference of Brazilian Symposium on Agroenergy and other 2 workshops.

The market potential assessment for solar hybrid cogeneration systems in Mexico was issued. The market value for the industrial and rural sector was discussed. The SOLHYCO technology can be deployed in Mexico. The market is very interesting, more than 2000 SOLHYCO units and investment of almost 800 million Euros were estimated. Also, a workshop on solar-hybrid cogeneration systems in Mexico was carried out.

Dissemination

For disseminating the technology, in the 4th reporting period a total of seven scientific papers concerning to the Solhyco applications were submitted and presented at conferences. Two further articles were released (press release, SolarPaces Annual report). The three Websites of the project in English, Portuguese and Spanish were updated.