Evaluation of Test Method for Large Solar Collector Efficiency

The Technical University of Denmark does research in large-scale solar collectors.

Abstract from DTU-homepage regarding the conference publication:

The test method of the standard EN12975-2 (European Committee for Standardization, 2004) is used by European test laboratories to determine the efficiency of solar collectors. In the test methods the mean solar collector fluid temperature in the solar collector, Tm is determined by the approximated equation where Tin is the inlet temperature to the collector and Tout is the outlet temperature from the collector. The specific heat of the solar collector fluid is in the test method as an approximation determined as a constant equal to the specific heat of the solar collector fluid at the temperature Tm. The power produced by the solar collector during a test period is determined by the product of the specific heat, the mass flow rate and the temperature increase of the solar collector fluid. The solar collector efficiency is in the standard determined by measurements at different temperature levels. Based on these efficiencies, an efficiency equation is determined by regression analysis. In the test method, there are no requirements on the ambient air temperature and the sky temperature. The paper will present an evaluation of the test method for a 12.5 m² flat plate solar collector panel from Arcon Solvarme A/S. The solar collector panel investigated has 16 parallel connected horizontal absorber fins. CFD (Computational Fluid Dynamics) simulations, calculations with a solar collector simulation program SOLEFF (Rasmussen and Svendsen, 1996) and thermal experiments are carried out in the investigation. The investigations elucidate: • How the mean solar collector fluid temperature Tm is underestimated by the approximated equation in the test standard and how the collector efficiency equation is influenced by the underestimation of Tm. The dependence of the volume flow rate is shown; • How the use of the approximated specific heat of the solar collector fluid is influencing the collector efficiency expression; • How the temperature levels used is influencing the collector efficiency expression; • How the measured collector efficiency is influenced by the weather conditions such as the ambient air temperature and the sky temperature. Based on the investigations, recommendations for change of the test methods and test conditions are considered. The investigations are carried out within the NEGST (New Generation of Solar Thermal Systems) project financed by EU.

Big players are waking up in Denmark

For the first time, a major district heating company is planing to adopt the concept of central solar heating. The company VEKS, distributing heat in the western part of the Copenhagen area is aiming at a CSHP with a solar collector size of 20,000 m2, placed along a main highway. The plant will match the current record of the Marstal plant, also placed in Denmark. The plant will deliver approx. the heat of 5-6,000 single family houses.CEO, Lars Gulev, of the “Vestegnens Kraftvarmeselskab” district heating company estimates the plant to produce 10 GWh heat to the district heating system. The goal is to get the plant into production before the International Climate Conference in Copenhagen, 2009, where the expectations are to replace the Kyoto-protocol with a more ambitious protocol.

It is expected that the application of central solar heating is a factor 4 more cost efficient compare to single family housing solar installations.

Relevant is to mention, that the company is expecting lower production prices for the solar plant compared to the traditional co-generation plants due to the facts that solar prices are reduced as conventional heating prices are increased during the later years.

“In the first place, the solar heating plant is planed due to commercial reasons. Getting reduced CO2 pollution is a environmental side effect.” says Lars Gulev.

We hope others will take this considerations up and CSHP will play a more dominant role in the future of district heating. It would be interesting to see, if surplus heat from the power production will be cooled into the see due to this investment.

You find information on VEKS’s homepage on http://www.veks.dk (in Danish), where you change to English and will be presented for general information on the district heating system of Copenhagen. The solar plant is presented at the location http://www.veks.dk/Artikler/Seneste%20nyt/Solvarme.aspx

Brænderstrup CSHP

The Danish Engineering Newspaper did present the following news about a new very large CSHP plant:

Place: Denmark, Brænderup (near Horsens), at the mainland.

Size: 8.000 m² solar collector field, sized to deliver 10% of the district heating demand.

Of the 13 million total cost, the 3.5 millioner DDK are funded by a public body, Energinet.dk, that last year did analyse solar and district heating lead to positiv economical results. This analysis pointed out that this is due to the fact that the heat- or power plant can be turned of for some periods, where the price for power is too low to produce.
The maximum effect is estimated to 6 MW heat, with an annual production of approx. 4,000 MWh heat, leading to a yearly emission reduction of 4,300 tons CO2. The plant is designed by consultant company Planenergi and Ramboll, Denmark with Arcon solar collector producer, the same companies as involved in most of the plants in Denmark.

Wikipedia and CSHP

To increase knowledge of large scale solar heating, I chose to place the facts into Wikipedia. Hereby, the probability of people picking up the technology, is improved a lot. You find the article by searching CSHP in the English version of Wikipedia.

I hope that the community will join me.

Wikipedia instead of this blog

I could write things in this blog. However I feel, that writing in Wikipedia has much wider impact. I will try to point you in the relevant directions. I can already say that there is no term such as Central or Large-Scale Solar Heating and Cooling in the Wikipedia at this time. So, I have to do some work there. You will se if I succeed in this task.

CSHP Hot topic?

Blogging is online. Hence online resources are rather streight forward to point at. However, Googling turns out to show a lack of knowledge within this topic: Central Solar Heating Plants (CSHP) or Large-Scale Solar Heating. As a researcher, library information are traditionally the main source for information. Therefore I tried to search in our library including more than 45,000 journal articles. The hit rate for is 22 hits that matches the full term “Central Solar Heating Plants”; “Central Solar Heating” gives 82 hits, below 10 hits unique and written after 2000, the newest from 2004, most much older.

Can we conclude that Central (large scale) Solar Heating is not relevant?

We can cerainly conclude that the topic is not “hot”. Is this because there is no research going on, is it because the research is not pulished or is not accepted for publishing? This question is keept open.

Is the topic relevant?

Positiv aspects of CSHP: From the experiences from Marstal and most other plants, one can conclude that the efficiency for large-scale systems is much highere, compared to small solar systems. The installation price is appoximatelly 60% of the installation price for small systems. The price-performance is hereby better for both price and performance. Details can be found in the Ph.D. citet earier in the blog by me. Maintenance, effieciency improvements and other aspects do also point at large installations.

Problematic aspects of CSHP: However, heat losses can be relativelly large, if one does not consider it appropriatelly. Modells to do that can be found, among others, in “Heat-load modelling for large systems”, Applied Energy, p. 371-387, 72(1), 2002. Another negative aspect of large scale solar heating comes up, when combining solar heating with co-production power plants, where the “waste” heat is applied in district heating. This is the case for the Copenhagen District Heating, where an area with up to 30 km in wide, supplied with district heating. The problem here is, that the heat exaustion by power production does make the replacement with solar problematic, as long as the overall efficiency of the power production and “waste heat” are related.

So, the question must be answered, that the topic is very relvant for any large-scale heating system. Solar cooling could be combined, improving the efficiency even more. If you are researchers and look for an important research topic, here it is. District Heating & Cooling supported by Solar Energy with and without long-term heat and cool storage. As you find in this blog, there is a lot to work with.

EuroSun 2003 Gothenburg with track on CSHP

At the EuroSun Conference 2003 in Gothenburg, Sweden, there was a track on District Heating and Renewables. The proceeding is online. I did not find the fulltext-proceeding though.

Michael Krause, Dr. Thesis in Optimization of Large Scale Solar District Heating

You can find the Dr.-thesis of Michael Krause, from the University of Kassel in Germany (in German). more later…

SunStore2 a EU-project enlarging the Marstal Plant

Sunstore 2 was the follow up end extention of the Marstal CSHP Plant. EU supported the project economically. You find the project survej on CORDIS.

Planenergi, a Danish Consultant Company was in charge for the project. You find a report on their website.

Uni Stuttgart took over

The University of Stuttgart took over our work with Marstal. Especially Thomas Schmidt was aktiv. One can find some of the work in the proceedings of the Futurestock konference.