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Papers by Göran Hellström

Upprättat; 2002; 20081216 (andbra

The design of a solar heating system using a duct store in the ground requires design procedures ... more The design of a solar heating system using a duct store in the ground requires design procedures that account for the different thermal processes involved. In order to obtain an accurate evaluation of the system heat balance, system performances have to be calculated with high resolution over several years, if not the lifetime of the system. Reliable and accurate simulation tools which use the TRNSYS programme have been developed for the simulation of Central Solar Heating Plants with Seasonal Storage (CSHPSS) using a ground duct store. The simulated systems involve a collector array, a short-term water buffer store, a ground duct store, an auxiliary heat supply and a heat distribution network to the consumer. The version of the duct heat storage model (DST) for TRNSYS has been improved based on the results of analyses performed with more detailed programmes. The flexibility of the TRNSYS programme makes the tools easily adaptable to a particular problem. The simulation tools are applied to a system defined for typical Swiss conditions. Optimal ratios between the main system parameters are sought for sizing purposes. The influence of the load size, load type (size of the proportion of domestic hot water) and load temperature levels are investigated. Several thousand simulations have been performed; these have helped to characterize and highlight important points in the design of such systems. Comparisons are made with systems having only a water store. They show that a duct store becomes economically more attractive for solar fraction larger than typically 50%, i.e. for seasonal heat storage. Corst reduction should be concentrated on cheaper collectors, as two thirds to three quarters of the solar cost is due to the collector array.

Upprättat; 2001; 20081216 (andbra

A new version of DST, the duct ground heat storage model, is developed for TRNSYS, a modular prog... more A new version of DST, the duct ground heat storage model, is developed for TRNSYS, a modular programme for the simulation of partial or complete energy systems. It combines the easy utilisation of the previous version for TRNSYS with the additional features of a more detailed version. The new improvements concern the local heat transfer along the flow path within the storage region. The modifications are based on detailed analyses of the thermal interaction between a solar collector field and a duct store. The DST model has been validated against measurements from several duct stores in operation. Detailed and time-consuming programmes were used to check the implementation of the new features. The present model is still reasonably fast and a large range of energy systems can be analysed for optimisation purposes.

To design borehole heat exchangers (BHE) for Ground Source Heat Pumps (GSHP) or Underground Therm... more To design borehole heat exchangers (BHE) for Ground Source Heat Pumps (GSHP) or Underground Thermal Energy Storage (UTES), the knowledge of underground thermal properties is paramount. In small plants (residential houses), these parameters usually are estimated. However, for larger plants (commercial GSHP or UTES) the thermal conductivity should be measured on site. A useful tool to do so is a thermal response test, carried out on a BHE in a pilot borehole (later to be part of the borehole field). For a thermal response test, basically a defined heat load is put into the hole and the resulting temperature changes of the circulating fluid are measured. Since late 1990s, this technology became more and more popular, and today is used routinely in many countries for the design of larger plants with BHEs, allowing sizing of the boreholes based upon reliable underground data. The paper includes a short description of the basic concept and the theory behind the thermal response test, looks at the history of its development, and emphasizes on the worldwide experience with this technology.

Applied Thermal Engineering, May 1, 2010

In design of ground-source energy systems the thermal performance of the borehole heat exchangers... more In design of ground-source energy systems the thermal performance of the borehole heat exchangers is important. In Scandinavia, boreholes are usually not grouted but left with groundwater to fill the space between heat exchanger pipes and borehole wall. The common U-pipe arrangement in a groundwaterfilled BHE has been studied by a three-dimensional, steady-state CFD model. The model consists of a 3 m long borehole containing a single U-pipe with surrounding bedrock. A constant temperature is imposed on the U-pipe wall and the outer bedrock wall is held at a lower constant temperature. The occurring temperature gradient induces a velocity flow in the groundwater-filled borehole due to density differences. This increases the heat transfer compared to stagnant water. The numerical model agrees well with theoretical studies and laboratory experiments. The result shows that the induced natural convective heat flow significantly decreases the thermal resistance in the borehole. The density gradient in the borehole is a result of the heat transfer rate and the mean temperature level in the borehole water. Therefore in calculations of the thermal resistance in groundwater-filled boreholes convective heat flow should be included and the actual injection heat transfer rate and mean borehole temperature should be considered.

$Research paper thumbnail of Influence on thermal response test by groundwater flow in vertical fractures in hard rock$

Renewable Energy, Nov 1, 2003

ABSTRACT

Energy, Jun 1, 2010

The use of ground-source heat pumps for heating and domestic hot water in dwellings is common in ... more The use of ground-source heat pumps for heating and domestic hot water in dwellings is common in Sweden. The combination with solar collectors has been introduced to reduce the electricity demand in the system. In order to analyze different systems with combinations of solar collectors and ground-source heat pumps, computer simulations have been carried out with the simulation program TRNSYS. Large differences were found between the system alternatives. The optimal design is when solar heat produces domestic hot water during summertime and recharges the borehole during wintertime. The advantage is related to the rate of heat extraction from the borehole as well as the overall design of the system. The demand of electricity may increase with solar recharging, because of the increased operating time of the circulation pumps. Another advantage with solar heat in combination with heat pumps is when the boreholes or neighbouring installations are drilled so close that they thermally influence each other. This may lead to decreasing temperatures in the ground, which gives decreased performance of the heat pump and increased use of electricity. The net annual heat extraction from the ground is reduced by recharge from solar heat.

Underground Thermal Energy Storage (UTES) applications have slowly gained acceptance on the Swedi... more Underground Thermal Energy Storage (UTES) applications have slowly gained acceptance on the Swedish energy market. Two UTES concepts are successfully implemented; the ATES (aquifer storage) and the BTES (borehole storage) systems. Also snow storage in pits or caverns has reached a commercial status. The number of ATES has steadily grown to 40 large-scale plants at the end of 2002. The systems are usually designed for cold storage in district cooling application, but industrial process cooling is also common. The economical potential in terms of straight payback time is usually very favourable. However, there is still a certain risk for operational problems that might jeopardize the calculated profit. Well clogging problems and system control remain as R&D issues to be solved. From a legislation point of view any ATES application needs a permit. The process of obtaining a permit has become complex and time-consuming since a new act on environmental assessment was put into effect in 1999. BTES systems are normally used in smaller applications. At the end of 2002 there were more than 200 installations comprising more than 10 boreholes. The majority of these are applied for space cooling of commercial or institutional buildings and for process cooling within the telecommunication sector. From a technical point of view, BTES are much simpler to construct and operate than ATES. Furthermore, they can be applied in almost any kind of geology. Another advantage compared to ATES is that the permitting procedure is much simpler. The major market obstacle is that the profitability is not always acceptable if calculated as a straight payback time. To increase the market potential, there is a need for further R&D on improvement of borehole heat exchangers and of more effective drilling methods. Snow storage is still a new technology though the Sundsvall snow storage plant has been operated successfully for several years. This good example has inspired several pre-studies of new snow storage plants. These have shown that snow storage is feasible in various sizes and in different applications.