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Articles


Direct solar thermal systems with thermosiphon frost protection and innovative control strategies using a Thermo-Differential Bypass Valve Direct solar thermal systems with thermosiphon frost protection and innovative control strategies using a Thermo-Differential Bypass Valve
June 2018 - PDF 0.21MB
By: Nico van Ruth
Publisher: EuroSun2018

This research involves a direct solar (combi-)system for use in cool climates, based on vacuum-tube collectors, that uses night-circulation for frost protection, which doesn’t rely on pumped circulation, but also provides frost protection through thermosiphon circulation as a back-up in case of power/control failures. Furthermore, this direct solar system does not use a collector sensor or storage tank sensor, but instead has both the temperature sensors integrated into the pumping station. These features are achieved by using a Thermo-Differential Bypass Valve, which is installed on the storage tank at the solar inlet, and which only opens when the solar supply temperature is higher than the storage tank temperature and bypasses the storage tank when the supply temperature is lower than the storage tank temperature. The solar circuit can thus be free of check valves, so it is free to thermosiphon when the pump is off, and the collector circuit can be regularly circulated to measure the collector temperature at the pumping station, without sacrificing system efficiency.

Polymer Collectors with Temperature Control - Potentials for System integration Polymer Collectors with Temperature Control - Potentials for System integration
October 2016 - PDF 2.05MB
By: Alexander Thür, Katarina Maslikova
Publisher: Gleisdorf SOLAR 2016

Within the Austrian research project SolPol-4/5 it is the goal to find solutions for solar thermal systems based on cheap polymer materials but with low temperature limits in order to realize significant cost reduction potentials. Therefore one major point is to keep the temperature of the solar collector (and the complete system) below the material limits which means below 100°C for cheap polymer materials. For this, several possibilities are under investigation in many research projects. One solution is to design the collector in such a way, that the performance does not allow stagnation temperatures above 100°C (temperature limited collector – TLC). Other solutions try to keep the collector performance highest possible during operation and reduce the performance during stagnation by different technical solutions (overheat controlled – OHC) like reduction of absorption characteristic at high temperatures (Föste, 2015), reduction of transmission of the transparent cover or increasing the heat losses by activating cooling processes like internal ventilation of the collector (Harrison, 2004) or using a thermosyphon driven backcooler (Thür, 2014). This simulation study based on different parameter variations estimates how different operating conditions can influence design parameters for a solar domestic hot water system (SDHW) with different collector types. For different possible market conditions, which can potentially be situated world-wide, the goal of these investigations is to find out dependencies of different design parameters depending on specific operating conditions for solar domestic hot water systems (SDHW).

Replacing traditional materials with polymeric materials in solar thermosiphon systems - Case study on pros and cons based on a total cost accounting approach Replacing traditional materials with polymeric materials in solar thermosiphon systems - Case study on pros and cons based on a total cost accounting approach
February 2016 - PDF 1.52MB
By: Bo Carlsson, Michaela Meir, John Rekstad, Dieter Preiss, Thomas Ramschak
Publisher: Solar Energy, Elsevier

The pros and cons of replacing traditional materials with polymeric materials in solar thermosiphon systems were analysed by adopting a total cost accounting approach. In terms of climatic and environmental performance, polymeric materials reveal better key figures than traditional ones like metals. In terms of present value total cost of energy, taking into account functional capability, end user investment cost, O&M cost, reliability and climatic cost, the results suggest that this may also be true when comparing a polymeric based thermosiphon system with a high efficient thermosiphon system of conventional materials for DHW production in the southern Europe regions. When present values for total energy cost are assessed for the total DHW systems including both the solar heating system and the auxiliary electric heating system, the difference in energy cost between the polymeric and the traditional systems is markedly reduced. The main reason for the difference in results can be related to the difference in thermal performance between the two systems. It can be concluded that the choice of auxiliary heating source is of utmost importance for the economical competiveness of systems and that electric heating may not be the best choice.

Novel solar thermal collector systems in polymer design – Part 3: aging behavior of PP absorber materials Novel solar thermal collector systems in polymer design – Part 3: aging behavior of PP absorber materials
2016 - PDF 0.43MB
By: Markus Povacz, Gernot M. Wallner, Michael K. Grabmann*, Susanne Beißmann, Klemens Grabmayer, Wolfgang Buchberger, Reinhold W. Lang
Publisher: Energy Procedia, Elsevier
A novel, accelerated aging test method was used to characterize the long-term stability of commercial black-pigmented polypropylene (PP) model materials for solar thermal absorbers at elevated temperatures. The PP model materials investigated, PP-B1 and PP-B2, are based on carbon black pigmented PP block copolymer grades. Using an automatized planning technique, sliced 100 µm thick specimens were prepared, aged in hot air and heat carrier fluid (mixture of 60 vol.-% deionized water and 40 vol.-% commercial propylene glycol) at 95°C, 115°C and 135°C for up to 15,000 hours, and characterized in terms of various aging indicators (i.e., remaining primary stabilizer content, oxidation temperature, carbonyl index and ultimate mechanical properties). In general two major trends were discerned. First, the aging processes of the PP compounds depend on the stabilizer system, but even more heavily on the interaction of the stabilizers with the carbon black pigments and the structure and morphology of the polymer. Although the compound PP-B2 exhibited much faster stabilizer loss and an associated drop in oxidation temperature than PP-B1, mechanical investigations proved a better long-term stability for PP-B2. Second, it was shown for the compounds investigated that exposure to hot air causes harsher aging than exposure to hot heat carrier fluid. This is, presumably related to the reduced quantity of dissolved oxygen and triazole-based corrosion inhibitors used in the heat carrier fluid. While PP-B1 is use for absorbers in unglazed collectors and overheating-protected glazed collectors, the investigations clearly revealed that PP-B2 is a promising alternative.
Novel solar thermal collector systems in polymer design – Part 5: Fatigue characterization of engineering PA grades for pressurized integrated storage collectors Novel solar thermal collector systems in polymer design – Part 5: Fatigue characterization of engineering PA grades for pressurized integrated storage collectors
2016 - PDF 0.94MB
By: Joerg Fischer*, Patrick R. Bradler, Mathias Schlaeger, Gernot M. Wallner, Reinhold W. Lang
Publisher: Energy Procedia, Elsevier
A novel aging test method considering the superimposed mechanical and environmental (temperature and environmental medium) loads representative for pressurized integrated storage collectors (ICS) is described. Engineering polyamide (PA) grades with short glass fiber (GF) reinforcement, which are of high relevance for endcaps of steel-pipe ICS absorbers or all-polymeric absorber/storage-tanks, are characterized on a specimen level. Therefore, specific test devices and test arrangements for fracture mechanics specimens with or without weld-line are implemented on an electro-dynamic test machine. Fatigue crack growth kinetics data are obtained by conducting cyclic mechanical loads under various environmental testing conditions. The experimental results of two glass-fiber reinforced PA grades, using compact type specimens, performed at two different temperatures (23 °C and 80 °C) and in two environmental media (air and water), are compared in terms of crack growth kinetics. Moreover, the influence of welding on the crack growth kinetics for one PA grade is shown. For all specimens (unwelded and welded) the fatigue crack growth rates are enhanced in water compared to air. In welded specimens the fatigue crack growth resistance is significantly reduced compared to unwelded specimens.
Polymeric materials in solar-thermal systems - performance requirements and loads Polymeric materials in solar-thermal systems - performance requirements and loads
2016 - PDF 1.91MB
By: Thomas Ramschak, Robert Hausner, Christian Fink
Publisher: Energy Procedia, Elsevier
A major basic problem in selecting appropriate polymeric materials and processing technology routes is related to the lack of well-defined functional and performance requirements on the component level and to material property requirements on the specimen level. Hence, in a first step several reference climate regions were defined for pumped systems (continental (Graz/Austria), moderate climate (Beijing/China)) and non-pumped systems (Mediterranean (Athens/Greece), hot and dry (Pretoria/South Africa), hot and humid (Fortaleza/Brazil)), respectively. For each of these reference regions various solar-thermal plant types (e.g., domestic hotwater systems for single family houses (pumped and thermosiphon); domestic hot-water systems for multi-family houses; solar combi-systems for domestic hot-water and space heating (pumped) were pre-defined and evaluated and optimized virtually by modelling and simulation. To determine performance requirements on the component level and to derive material property requirements on the specimen level all-purpose modelling and design tools for collectors were implemented and used which allow for the description of temperature profiles, stagnation conditions, efficiency curves, pressure losses, distribution of fluid and heat flow and the thermal and hydraulic optimisation of the whole collector.
Lifetime modeling of polypropylene absorber materials for overheating protected hot water collectors Lifetime modeling of polypropylene absorber materials for overheating protected hot water collectors
January 2016 - PDF 0.48MB
By: G.M. Wallner, M. Povacz, R. Hausner, R.W. Lang
Publisher: Solar Energy, Elsevier

For the utilization of polymeric materials in high-demanding applications like solar thermal systems it is of utmost importance to define the performance requirements and to investigate the applicability of components for defined systems under service relevant conditions. This paper deals with the lifetime estimation of black-pigmented polypropylene (PP) absorber grades for overheating protected solar thermal collector systems for hot water preparation in five representative climate zones. Based on experimental aging data in hot air and heat carrier fluid at elevated temperatures (95 °C, 115 °C and 135 °C) and climatic input data, as well as deduced loading conditions and absorber temperature distributions, the lifetime was calculated using a theoretical and an empirical extrapolation approach and assuming cumulating damages in service relevant temperature intervals. Depending on the PP grade, the extrapolation method and the location, endurance limits ranging from 8 to 50 years were obtained. The PP grade with ß-spherulithic structures and less carbon black exhibited a superior performance (factor 2) compared to a well-established grade which is currently widely used for swimming pool absorbers.

A total cost perspective on use of polymeric materials in solar collectors - Importance of environmental performance on suitability A total cost perspective on use of polymeric materials in solar collectors - Importance of environmental performance on suitability
July 2014 - PDF 0.71MB
By: Bo Carlsson, Helena Persson, Michaela Meir, John Rekstad
Publisher: Applied Energy, Elsevier

To assess the suitability of solar collector systems in which polymeric materials are used versus those in which more traditional materials are used, a case study was undertaken. In this case study a solar heating system with polymeric solar collectors was compared with two equivalent but more traditional solar heating systems: one with flat plate solar collectors and one with evacuated tube solar collectors. To make the comparison, a total cost accounting approach was adopted. The life cycle assessment (LCA) results clearly indicated that the polymeric solar collector system is the best as regards climatic and environmental performance when they are expressed in terms of the IPPC 100 a indicator and the Ecoindicator 99, H/A indicator, respectively. In terms of climatic and environmental costs per amount of solar heat collected, the differences between the three kinds of collector systems were small when compared with existing energy prices. With the present tax rates, it seems unlikely that the differences in environmental and climatic costs will have any significant influence on which system is the most favoured, from a total cost point of view. In the choice between a renewable heat source and a heat source based on the use of a fossil fuel, the conclusion was that for climatic performance to be an important economic factor, the tax or trade rate of carbon dioxide emissions must be increased significantly, given the initial EU carbon dioxide emission trade rate. The rate would need to be at least of the same order of magnitude as the general carbon dioxide emission tax rate used in Sweden. If environmental costs took into account not only the greenhouse effect but also other mechanisms for damaging the environment as, for example, the environmental impact factor Ecoindicator 99 does, the viability of solar heating versus that of a natural gas heating system would be much higher.

Simulation of a solar collector array consisting of two types of solar collectors, with and without convection barrier Simulation of a solar collector array consisting of two types of solar collectors, with and without convection barrier
2014 - PDF 0.54MB
By: Federico Bava*, Simon Furbo, Bengt Perers
Publisher: Energy Procedia, Elsevier
The installed area of solar collectors in solar heating fields is rapidly increasing in Denmark. In this scenario even relatively small performance improvements may lead to a large increase in the overall energy production. Both collectors with and without polymer foil, functioning as convection barrier, can be found on the Danish market. Depending on the temperature level at which the two types of collectors operate, one can perform better than the other. This project aimed to study the behavior of a 14 solar collector row made of these two different kinds of collectors, in order to optimize the composition of the row. Actual solar collectors available on the Danish market (models HT-SA and HT-A 35-10 manufactured by ARCON Solar A/S) were used for this analysis. To perform the study, a simulation model in TRNSYS was developed based on the Danish solar collector field in Braedstrup. A parametric analysis was carried out by modifying the composition of the row, in order to find both the energy and economy optimum.
© 2015 The Authors. Published by Elsevier Ltd.
Peer-review by the scientific conference committee of SHC 2014 under responsibility of PSE AG.
Entwicklung eines Verfahrens für die Wirtschaftlichkeitsberechnung solarthermischer Anlagen: die LCOH Methode
By: Y. Louvet, S. Fischer, S. Furbo, F. Giovannetti, F. Mauthner, D. Mugnier, D. Philippen, K. Vajen
Publisher: 27. Symposium Thermische Solarenergie, Kloster Banz, 10.-12. Mai 2017

Ende 2015 began der Task 54 des Solar Heating and Cooling Programms der Inter-nationalen Energieagentur (IEA-SHC Task 54), Price reduction of solar thermal sys-tems. Ziele des Tasks sind Preisesenkungspotenziale für Solar thermische Anlagen zu identifizieren und konkreten technischen und wirtschaftlichen Pfade vorzuschla-gen, um die solaren Wärmegestehungskosten um bis zu 40 % zu reduzieren /1/. Kostensenkungen sind von entscheidender Bedeutung für die Branche, die seit ein paar Jahren mit einem Rückgang der Verkaufszahlen auf dem europäischen Markt konfrontiert ist. Im Rahmen des Tasks wurde daher ein transparentes Verfahren zur Wirtschaftlichkeitsberechnung und eine Kennzahl entwickelt werden, um das Preis-senkungspotenzial unterschiedlicher Lösungen miteinander vergleichen zu können.