| 1. |
- Gehlin, S E A, et al.
(författare)
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Influence on thermal response test by groundwater flow in vertical fractures in hard rock
- 2003
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Ingår i: Renewable Energy. - 0960-1481 .- 1879-0682. ; 28:14, s. 2221-2238
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Tidskriftsartikel (refereegranskat)abstract
- In this paper different approaches to groundwater flow and its effect in the vicinity of a borehole ground heat exchanger are discussed. The common assumption that groundwater flow in hard rock may be modelled as a homogeneous flow in a medium with an effective porosity is confronted and models for heat transfer due to groundwater flow in fractures and fracture zones are presented especially from a thermal response test point of view. The results indicate that groundwater flow in fractures even at relatively low specific flow rates may cause significantly enhanced heat transfer, although a continuum approach with the same basic assumptions would suggest otherwise. (C) 2003 Elsevier Science Ltd. All rights reserved.
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| 2. |
- Gehlin, S E A, et al.
(författare)
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The influence of the thermosiphon effect on the thermal response test
- 2003
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Ingår i: Renewable Energy. - 0960-1481 .- 1879-0682. ; 28:14, s. 2239-2254
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Tidskriftsartikel (refereegranskat)abstract
- The issue of natural and forced groundwater movements, and its effect on the performance of ground heat exchangers is of great importance for the design and sizing of borehole thermal energy systems (BTESs). In Scandinavia groundwater filled boreholes in hard rock are commonly used. In such boreholes one or more intersecting fractures provide a path for groundwater flow between the borehole and the surrounding rock. An enhanced heat transport then occurs due to the induced convective water flow, driven by the volumetric expansion of heated water. Warm groundwater leaves through fractures in the upper part of the borehole while groundwater of ambient temperature enters the borehole through fractures at larger depths. This temperature driven flow is referred to as thermosiphon, and may cause considerable increase in the heat transport from groundwater filled boreholes. The thermosiphon effect is connected to thermal response tests, where the effective ground thermal conductivity is enhanced by this convective transport. Strong thermosiphon effects have frequently been observed in field measurements. The character of this effect is similar to that of artesian flow through boreholes. (C) 2003 Elsevier Science Ltd. All rights reserved.
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| 3. |
- Hellström, Bengt, et al.
(författare)
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The impact of optical and thermal properties on the performance of flat plate solar collectors
- 2003
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Ingår i: Renewable Energy. - 0960-1481. ; 28:3, s. 331-344
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Tidskriftsartikel (refereegranskat)abstract
- The impact of the optical properties on the annual performance of flat plate collectors in a Swedish climate has been estimated with the MINSUN program. The collector parameters were determined with a theoretically based calculation program verified from laboratory measurements. The importance of changes in solar absorptance and thermal emittance of the absorber, the addition of a teflon film or a teflon honeycomb, antireflection treatment of the, cover glazing and combinations of these improvements were investigated. The results show that several improvements can be achieved for solar thermal absorbers. A combined increase in absorptance from 0.95 to 0.97 and a decrease in emittance from 0.10 to 0.05 increase the annual performance with 6.7% at 50 degreesC operating temperature. The increase in performance by installing a teflon film as second glazing was estimated to 5.6% at 50 degreesC. If instead a teflon honeycomb is installed, a twice as high performance increase is obtained, 12.1 %. Antireflection treatment of the cover glazing increases the annual output with 6.5% at 50 degreesC. A combination of absorber improvements together with a teflon honeycomb and an antireflection treated glazing results in a total increase of 24.6% at 50 degreesC. Including external booster reflectors increases the expected annual output at 50 degreesC to 19.9-29.4% depending on reflector material. (C) 2002 Elsevier Science Ltd. All rights reserved.
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| 4. |
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| 5. |
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| 6. |
- Adsten, Monika, et al.
(författare)
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The influence of climate and location on collector performance
- 2002
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Ingår i: Renewable energy. - 0960-1481 .- 1879-0682. ; 25:4, s. 499-509
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Tidskriftsartikel (refereegranskat)abstract
- The influence of annual climate variations on the performance of solar thermal collectors in the northern part of Europe has been investigated. The annual solar collector energy output has been calculated with the MINSUN simulation program using hourly, measured climatic data for the years 1983–98 for three cities situated in the south (Lund), central (Stockholm) and north (Luleå) of Sweden. A synthetic year created with the Meteonorm weather simulation program was also used in the simulations. Two solar thermal collectors were modelled: a flat plate solar collector and a tubular vacuum collector, both of commercial standard.The thermal energy output is strongly correlated to the annual global irradiation at a horizontal surface. The annual average energy delivered from the flat plate collector was 337 kWh/m2 for Stockholm (337 for Lund and 298 for Luleå), and from the vacuum tube collector 668 kWh/m2 for Stockholm (675 for Lund and 631 for Luleå) at an operating temperature of T=50°C. Maximum deviations from the average value for this 16-year period are around 20% for the flat plate and 15% for the vacuum tube collector, at T=50°C.The relation between global irradiation on a horizontal surface and the annually collected thermal energy at a constant operating temperature could be fitted to a linear equation: qu=aG(0°)+bT, where qu is the energy output from the collector, G(0°) the global irradiation at a horizontal surface, T the average temperature of the collector fluid, and a and b fitting parameters in a double linear regression analysis.
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| 7. |
- Gustavsson, Mathias, 1969, et al.
(författare)
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The impact of solar home systems on rural livelihoods. Experiences from the Nyimba Energy Service Company in Zambia
- 2004
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Ingår i: Renewable Energy. - : Elsevier BV. - 0960-1481. ; 29:7, s. 1059-1072
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Tidskriftsartikel (refereegranskat)abstract
- An Energy Service Company (ESCO) in the town of Nyimba, Zambia, has been operating 100 solar home systems since the year 2000. The company is part of a pilot project implemented by the Government of Zambia with the aim to apply the ESCO concept for diffusion of solar technology. The change in livelihood as a result of the access to electric services has been investigated in a survey. Clients were satisfied with the services they receive, although they are paying more than previously for energy. Light hours did not increase, but the quality of light was improved, enabling activities such as domestic work at night and studying for longer hours. Many of the clients had acquired TV and video, and become part of a more global culture. Appliances for entertainment, such as video and TV, were considered more attractive than productive appliances such as a water pump by many respondents. In some cases also people from households without solar installations benefited from the installations, for instance, children gathering in houses with solar to study at night.
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| 8. |
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| 9. |
- Lindberg, Eva, et al.
(författare)
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An animation tool for demonstrating the importance of edge filters in thermophotovoltaic applications
- 2002
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Ingår i: Renewable energy. - : Elsevier Science Ltd. - 0960-1481 .- 1879-0682. ; 28, s. 1305-1315
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Tidskriftsartikel (refereegranskat)abstract
- In order to achieve high efficiency in a thermophotovoltaic (TPV) generator, it is important that a high fraction of emitted photons with energies below the TPV cell bandgap are reflected back to the emitter. This can be accomplished in several ways, and one suggestion is to place an edge filter between the emitter and the TPV cell array in an elliptical optic design. An animation tool, developed in the Excel® program, for determining the efficiency of an optical system is presented. The animation components are a black body emitter, an edge filter, and an array of TPV cells. The tool has been used to demonstrate the importance of an efficient filter and the usefulness of optics that makes the edge of the filter as sharp as possible.
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| 10. |
- Lindberg, Eva, et al.
(författare)
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An animation tool for demonstrating the importance of edge filters in thermophotovoltaic applications
- 2003
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Ingår i: Renewable energy. - 0960-1481 .- 1879-0682. ; 28, s. 1305-1315
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Tidskriftsartikel (refereegranskat)abstract
- In order to achieve high efficiency in a thermophotovoltaic (TPV) generator, it is important that a high fraction of emitted photons with energies below the TPV cell band gap are reflected back to the emitter. This can be accomplished in several ways, and one suggestion is to place an edge filter between the emitter and the TPV cell array in an elliptical optic design. An animation tool, developed in the Excel program, for determining the efficiency of an optical system is presented. The animation components are a black body emitter, an edge filter, and an array of TPV cells. The tool has been used to demonstrate the importance of an efficient filter and the usefulness of optics that makes the edge of the filter as sharp as possible. It is available at www.du.se/tpv.
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