Göran Hellström

High temperature solar heated seasonal storage system for low temperature heating of buildings

A preliminary study of a solar-heated low-temperature space-heating system with seasonal storage in the ground has been performed. The system performance has been evaluated using the simulation models TRNSYS and MINSUN together with the ground storage module DST. The study implies an economically feasible design for a total annual heat demand of about 2500 MWh. The main objective was to perform a study on Anneberg, a planned residential area of 90 single-family houses with 1080 MWh total heat demand. The suggested heating system with a solar fraction of 60% includes 3000 m2 of solar collectors but electrical heaters to produce peak heating. The floor heating system was designed for 30°C supply temperature. The temperature of the seasonal storage unit, a borehole array in crystalline rock of 60,000 m3, varies between 30 and 45°C over the year. The total annual heating costs, which include all costs (including capital, energy, maintenance etc.) associated with the heating system, were investigated for three different systems: solar heating (1000 SEK MWh−1), small-scale district heating (1100 SEK MWh−1) and individual ground-coupled heat pumps (920 SEK MWh−1). The heat loss from the Anneberg storage system was 42% of the collected solar energy. This heat loss would be reduced in a larger storage system, so a case where the size of the proposed solar heating system was enlarged by a factor of three was also investigated. The total annual cost of the solar heating system was reduced by about 20% to about 800 SEK MWh−1, which is lower than the best conventional alternative.

Multipole method to calculate borehole thermal resistances in a borehole heat exchanger

Ground-source heat pump systems use borehole heat exchangers to transfer heat to and from the ground. An important feature is the local thermal resistances between the heat carrier flow channels in the borehole and the surrounding ground. The counter-flow heat exchange between the pipes is also important, particularly for the axial temperature variation. These resistances can be represented by a thermal network between the pipes and the ground. The borehole thermal resistance is readily obtained from the network. A fairly intricate mathematical algorithm, the multipole method, to compute the temperature fields and, in particular, the thermal resistances is presented. This article focuses on the application of the model, leaving the detailed mathematics to a background report. The formulas and methodology required for any particular case are presented in detail. The multipole method gives a solution with very high, and easily verified, accuracy for the steady-state heat conduction in a region perpendicular to the borehole axis. It is fairly straightforward to implement the algorithm in any design software. The computational time requirements are negligible.

On the use of the diffusion equation in test case 6 of DECOVALEX

Test case 6 of DECOVALEX addresses the hydromechanical behavior of fractured crystalline rocks submitted to high pressure testing between of inflatable packers. The objective of the present study is to analyze the possibilities of the diffusion equation to account that the flow rate increases with time when a constant pressure is maintained between the borehole packers.

Forced Convective-Diffusive Heat Flow in Insulations: A New Analytical Technique Applied to Air Leakage through a Slit

Forced convective-diffusive heat flow in porous insulation materials is governed by an equation for the air pressure distribution with an ensuing air flow and a heat equation with convection and diffusion. The pressure equation for a ho mogeneous material may be solved analytically with simple geometries and bound ary conditions. The new technique uses a transformed form of the convective-diffusive equation for which the awkward first-order derivatives of the temperature (the convective part) are removed. New explicit solutions for certain two-dimensional, steady-state cases may be derived. The considered example concerns air leakage through an insulation which is open on one side and airtight on the other except for an open slit. Air infiltrates through the slit and leaves through the open side. The solution gives the complete pressure and temperature fields. The extra heat loss due to air leakage is given by an explicit expression which contains only a single, dimensionless parameter.

Conformal Flow Models for Warm and Cold Storage in Aquifers

The paper presents the thermohydraulic models for aquifer thermal energy storage developed by the Lund Group for Ground Heat. The basic assumption is that the aquifer is horizontally homogeneous an ...