Picture Credit: www.hollidayheating.com

Renewable energy sources have one thing in common; they all existed before man appeared on this planet. Wind, wave, hydro, solar, geothermal and tidal power are all forces of nature and are mostly intermittent energy sources, geothermal is the only consistent phenomenon. Geothermal renewable energy sources where probably the first to be fully utilised by man. Early civilisations tapped this heat to cook, fire clay pottery, create baths and spas and even heat their homes. Roman villas had under floor heating from natural hot springs over 2000 years ago.

Shallow geothermal resources (<400 m depth by governmental definition in several countries) are omnipresent. Below 15-20 m depth, everything is geothermal. Figure 1 show a summary of the soil thermal properties. The temperature difference between the ground and the fluid in the ground heat exchanger drives the heat transfer. So, it is important to determine the ground temperature. The temperature field is governed by terrestrial heat flow and the local ground thermal conductivity structure (groundwater flow). In some countries, all energy stored in form of heat beneath the earth surface is per definition perceived as geothermal energy. The same approach is used in North America. The ubiquitous heat content of shallow resources can be made accessible either by extraction of groundwater or, more frequent, by artificial circulation like the borehole heat exchanger (BHE) system. This means, the heat extraction occurs–in most cases–by pure conduction; there is no formation fluids required. The most popular BHE heating system with one of more boreholes typically 50-200 m deep is a closed circuit, heat pump coupled system, ideally suited to supply heat to smaller, de-central objects like single family or multi-family dwellings (Figure 2). The heat exchangers (mostly double U-tube plastic pipes in grouted boreholes) work efficiently in nearly all kinds of geologic media (except in material with low thermal conductivity like dry sand or dry gravel). This means to tap the ground as a shallow heat source comprise:

• Groundwater wells (“open” systems),
• Borehole heat exchangers (BHE),
• Horizontal heat exchanger pipes (including compact systems with trenches, spirals, etc.), and
• ‘Geo-structures’ (foundation piles equipped with heat exchangers).

Figure 1: Measured thermal properties for different soils
Figure 2: Ground source heat pumps

A common feature of these ground-coupled systems is a heat pump, attached to a low-temperature heating system like floor panels/slab heating. They are all termed “ground-source heat pumps” (GSHP) systems. In general, these systems can be tailored in a highly flexible way to meet locally varying demands. Experimental and theoretical investigations (field measurement campaigns and numerical model simulations) have been conducted over several years to elaborate a solid base for the design and for performance evaluation of BHE systems. While in the 80s, theoretical thermal analysis of BHE systems prevailed in Sweden monitoring and simulation was done in Switzerland, and measurements of heat transport in the ground were made on a test site in Germany.

In the German test system at Schöffengrund-Schwalbach near Frankfurt/Main, a 50-m BHE was surrounded by a total of 9 monitoring boreholes at 2.5, 5 and 10 m distance, also 50 m deep. Temperatures in each hole and at the BHE itself were measured with 24 sensors at 2 m vertical distance, resulting in a total of 240 observation locations in the underground. This layout allowed investigating the temperature distribution in the vicinity of the BHE. The influence from the surface is visible in the uppermost approximately 10 m (Figure 1), as well as the temperature decrease around the BHE at the end of the heating season. Measurements from this system were used to validate a numerical model for convective and conductive heat transport in the ground. Starting in 1986, an extensive measurement campaign has been performed at a commercially delivered BHE installation in Elgg near Zurich. The object of the campaigns is a single, coaxial, 10 m long BHE in use since its installation in a single-family house. The BHE supplies a peak thermal power of about 70 W per m of length.

Heat Exchanger Design

A heat exchanger is usually referred to as a micro heat exchanger (μHX) if the smallest dimension of the channels is at the micrometer scale, for example from 10 μm to 1 mm. Beside the channel size, another important geometric characteristic is the surface area density ρ (m2/m3), which is defined as the ratio of heat exchange surface area to volume for one fluid. It reflects the compactness of a heat exchanger and provides a classification criterion of note that the two parameters, the channel size and surface area density, are interrelated, and the surface area density increases when the channel size decreases. The exchangers that have channels with characteristic dimensions of the order of 100 μm are likely to get an area density over 10 000 m2/m3 and usually referred to as μHXs.

By introducing α in the specific heat exchanger performance equation, the volumetric heat transfer power P/V (W/m3) can be expressed as follows:

P = FUA ΔTm= FUA ρ α V ΔTm (1)
P/ V= ρ FU ΔTm (2)

Where, U, ΔTm and F refer to the overall heat transfer coefficient (W/m2 K), the mean temperature difference (K) and the dimensionless mean temperature difference correction factor for flow configuration respectively. Note that for a specific heat exchanger performance, high values of α lead to a corresponding high volumetric heat transfer power, larger than that of the conventional equipment by several orders of magnitude. As a result, the heat exchanger design by miniaturisation technology has become a common research focus for process intensification.

The main advantages of the μHX design are its “compactness, effectiveness and dynamic”. These properties enable an exact process control and intensification of heat and mass transfer:

Compactness: The high surface area density reduces substantially the volume of the heat exchanger needed for the same thermal power. As a result, the space and costly material associated with constructing and installing the heat exchanger could be reduced significantly. Moreover, the fluid holdup is small in a μHX; this is important for security and economic reasons when expensive, toxic, or explosive fluids are involved.
Effectiveness: The relatively enormous overall heat transfer coefficient of the μHXs makes the heat exchange procedure much more effective. In addition, the development of microfabrication techniques such as LIGA, stereolithography, laser beam machining, and electroformation allows designing a μHX with more effective configurations and high-pressure resistance.
Dynamic: The quick response time of a μHX provides a better temperature control for relatively small temperature differences between fluid flows. The quick response (small time constant) is connected to the small inertia of the heat transfer interface (the small metal thickness that separates the two fluids). On the other hand, the exchanger as a whole, including the “peripheric” material, usually has a greater inertia than conventional
exchangers, entailing a large time-constant. Thus, the response of one fluid to a temperature change of the other fluid comprises two “temperature-change waves”, with very distinct time-constants. In conventional exchangers, it is possible that the two responses are blurred into one.

However, the μHXs on one hand, the high performance is counterbalanced by a high pressure drop, a rather weak temperature jump and an extremely short residence time. On the other hand, those fine channels (~100 μm) are sensitive to corrosion, roughness and fouling of the surfaces. Moreover, the distinguishing feature of the μHXs is their enormous volumetric heat exchange capability accompanied with some difficulties in achievement realisation. The μHXs design optimisation lies, on one hand, in maximising the heat transfer in a given volume taking place principally in microchannels, while, on the other hand, minimising the total pressure drops, the dissipations, or the entropy generation when they function as a whole system. Moreover, difficulties such as the connection, assembly, and uniform fluid distribution always exist, all of which should be taken into account at the design stage of the μHXs. All these make the optimisation of the μHXs design a multi-objective problem, which calls for the introduction of multi-scale optimisation method to bridge the microscopic world and the macroscopic world. In recent years, the fractal theory and constructal theory have been introduced to bridge the characteristics of heat and mass transfer that mainly takes place at the micro-scale and the global performance of the heat exchanger system.

The concept of multi-scale heat exchanger is expected to have the following characteristics:

• A relatively significant specific heat exchange surface compared to that of traditional exchangers;
• A high heat transfer coefficient, as heat transfer takes place at micro-scales and meso-scales;
• An optimised pressure drop equally distributed between the various scales;
• A modular character, allowing assembly of a macro-scale exchanger from microstructured modules.

Some difficulties still exist. On one hand, the properties of flow distribution in such an exchanger are still unknown. A lot of research work still needs to be done for the equidistribution optimisation. On the other hand, 3-D modelling of heat transfer for such an exchanger requires a thorough knowledge of the hydrodynamics and profound studies on elementary volume (smallest scale micro channels). Finally, maintenance problems for this type of integrated structures may become unmanageable when fouling; corrosion, deposits or other internal perturbations are to be expected. Figures 3-5 show the connections of the heat exchanger, water pump, heat rejection fan and expansion valve.

Figure 3: The heat exchanger
Figure 4: The connections of the heat exchanger, water pump, heat rejection fan and expansion valve
Figure 5: The connections of the heat exchanger and expansion valve

The present DX GSHP system has been designed taking into account the local meteorological and geological conditions and then systems was installed, using the ground source as a heat source. This project yielded considerable experience and performance data for the novel methods used to exchange heat with the primary effluent.

The heat pump has also fitted in dry, well-ventilated position where full access for service was possible and monitored the performance of a number of the DX GSHPs, including one so-called ‘hybrid’ system that included both ground-coupling and a cooling tower.

The GSHPs provide an effective and clean way of heating buildings worldwide. They make use of renewable energy stored in the ground, providing one of the most energy-efficient ways of heating buildings. They are suitable for a wide variety of building types and are particularly appropriate for low environmental impact projects. They do not require hot rocks (geothermal energy) and can be installed in most of the world, using a borehole or shallow trenches or, less commonly, by extracting heat from a pond or lake. Heat collecting pipes in a closed loop, containing water (with a little antifreeze) are used to extract this stored energy, which can then be used to provide space heating and domestic hot water. In some applications, the pump can be reversed in summer to provide an element of cooling. The only energy used by the GSHP systems is electricity to power the pumps. Typically, a GSHP will deliver three or four times as much thermal energy (heat) as is used in electrical energy to drive the system. For a particularly environmental solution, green electricity can be purchased. The GSHP systems have been widely used in other parts of the world, including North America and Europe, for many years. Typically, they cost more to install than conventional systems; however, they have very low maintenance costs and can be expected to provide reliable and environmentally friendly heating for in excess of 20 years. Ground source heat pumps work best with heating systems, which are optimised to run at a lower water temperature than is commonly used in the UK boiler and radiator systems. As such, they make an ideal partner for underfloor heating systems.

T1 is the Heat exchanger temperature
T2 is the compressor temperature
T3 is the condenser temperature
T4 is the vapour temperature
T5 is the indoor temperature
T6 is the pit temperature

Figures 6-8 show daily system temperatures for a sample day in each period and the periods of operation of the auxiliary heater and the immersion heater. The performance of the heat pump is inversely proportional to the difference between the condensation temperature and the evaporation temperature (the temperature lift). Figure 9 shows the output of the heat pump for a range of output (condensation) temperatures. These are stable operating conditions, but not true steady state conditions. At output temperatures greater than 40°C, the heat pump was providing heating to the domestic hot water. The scatter in the points is largely due to variations in the source temperatures (range 0.2°Cto 4.3°C). These results indicate that the system performance meets and possibly exceeds the specified rating for the heat pump of 3.7 kW at an output temperature of 45°C. Two different control mechanisms for the supply of energy from the heat pump for space heating were tested.

Figure 6: Variation of temperatures for heat exchanger for two weeks
Figure 7: Variation of temperatures for heat exchanger for 45 days
Figure 8: Variation of temperatures for heat exchanger for year
Figure 9: Variation of heat pump output with temperature

From March 2014 until July 2015, the supply of energy from the heat pump to the space heating system was controlled by a thermostat mounted in the room. From August 2008, an alternative control using an outside air temperature sensor was used. This resulted in the heat pump operating more continuously in cold weather and in reducing the use of the auxiliary heater considerably. The amount the auxiliary heater is used has a large effect on the economic performance of the system. Using the outdoor air temperature sensor will result in the adjustment of the return temperature being for changes in the outdoor temperature and a good prediction of the heating requirement. Very stable internal temperatures were maintained. Figure 9 shows the daily total space heating from the heat pump and the auxiliary heater for the two heating control systems. The same period of the year has been compared, using the room temperature sensor and an outdoor air temperature sensor. The operating conditions were not identical, but the average 24-hour temperatures for the two periods were quite similar at 9.26°C and 9.02°C respectively.

Performance of the Ground Collector

The flow rate in the ground coil is 0.23 l/s. The heat collection rate varies from approximately 19 W to 27 W per meter length of collector coil. In winter, the ground coil typically operates with a temperature differential of about 5°C (i.e., a flow temperature from the ground of 2°C to 3°C and a return temperature to the ground coil of -1°C to -2°C). Icing up of the return pipework immediately below the heat pump can be quite severe. The ground coil temperatures are considerably higher in summer when, for water heating, the temperature differential is similar but flow and return temperatures are typically 11°C and 6°C respectively. When the heat pump starts, the flow and return temperatures stabilise very quickly. Even over sustained periods of continuous operation the temperatures remained stable. The ground coil appears adequately sized and could possibly be oversized. Figure 9 shows the variation of ground source heat pump against ground temperatures.

Geothermal Energy: Electricity Generation and Direct Use at the End 2008

Concerning direct heat uses, Table 1 shows that the three countries with the largest amount of installed power: USA (5,366 MWt), China (2,814 MWt) and Iceland (1,469 MWt) cover 58% of the world capacity, which has reached 16,649 MWt, enough to provide heat for over 3 million houses.

A residential GSHP system is more expensive to install than a conventional heating system. It is most cost-effective when operated year-round for both heating and cooling. In such cases, the incremental payback period can be as short as 3–5 years. A GSHP for a new residence will cost around 9-12% of the home construction costs. A typical forced air furnace with flex ducting system will cost 5-6% of the home construction costs. Stated in an alternative form, the complete cost of a residential GSHP system is $3,500-$5,500 per ton. Horizontal loop installations will generally cost less than vertical bores. For a heating dominated residence, figure around 550 square feet/ton to size the unit. A cooling dominated residence would be estimated around 450 square feet/ton. The Table 2 compares three types of systems.

Out of about 60 countries with direct heat plants, beside the three above-mentioned nations, Turkey, several European countries, Canada, Japan and New Zealand have sizeable capacity.

The GSHPs energy cost savings vary with the electric rates, climate loads, soil conditions, and other factors. In residential building applications, typical annual energy savings are in the range of 30 to 60 percent compared to conventional HVAC equipment.

Most systems have less than 15 kWth heating output, and with ground as heat source, direct expansion systems are predominant. Ground-source heat pumps had a market share of 95% in 2016 (Figure 10). Figure 11 illustrates the monthly energy consumption for a typical household in the United Kingdom. Unlike air source units, GSHP systems do not need defrost cycles nor expensive backup electric resistance heat at low outdoor air temperatures. The stable temperature of a ground source is a tremendous benefit to the longevity and efficiency of the compressor.

Figure 10: Distribution of heat sources for heat pumps (for space heating)
Figure 11: Monthly heating energy demands

The energy used to operate this pump could be reduced if it were controlled to operate only when the heat pump was supplying heat. The improvement in efficiency would be greatest in summer when the heat pump is only operating for a short period each day. If this pump were controlled to operate only when the heat pump is operating, it is estimated that the overall annual performance factor of the heat pump system would be 3.43, and that the average system efficiencies for the period November to March and April to September would be 3.42 and 3.44 respectively.

Under these conditions, it is predicted that there would only be a small variation in the efficiency of the heat pump system between summer and winter. This is explained by the fact that although the output temperature required for domestic water heating is higher than that required for space heating, the ground temperatures are significantly higher in the summer than in the winter.

There is clearly a lot more that must be done to support distribution GSHPs in general especially from the perspective of buildings in the planning and operation, and distribution GSHP systems (Figures 12-14).

Figure 12: Residential energy consumptions according to end use
Figure 13: Commercial energy consumptions according to end use
Figure 14: Energy consumptions according to end use

The key to the diffusion of any innovation is the ability to reduce the uncertainty or risk associated with the innovation. There are several diffusion attributes of a technology that help us identify the technology’s ability to overcome uncertainty and achieve potential adoption. The key attributes have been divided into five categories, presented below with our assessment of the status of GSHP relative to these attributes (Table 3).

Applications of Geoexchanger Systems

Geoexchanger energy is a natural resource, which can be used in conjunction with heat pumps to provide energy for heating and hot water. CO2 emissions are much lower than gas fired boilers or electric heating systems. Geothermal heating is more expensive to install initially, than electrical or gas fired heating systems. However, it is cheaper to run, has lower maintenance costs, and is cleaner in use than other sources of heating.

The temperature of the earth under 2 metres of the surface is a fairly constant 10ºC throughout the year. At a depth of about 100 metres, the temperature of any water or rock is at about 12ºC throughout the year. The heat stored at this depth comes largely from the sun, the earth acting as a large solar collector. For very deep wells, in excess of about 170 metres, there is an added component of heat from the core of the earth. As an approximation, one can add 3ºC of heat gain for every 100 metres of depth drilled into the earth.

A closed loop system takes the heat gained from the bedrock itself. In a vertical system a borehole of a diameter of about 150mm is drilled, depth varies between 32 and 180 metres but will depend on the energy requirements. Multiple boreholes can be drilled. A pair of pipes with a special U-bend assembly at the bottom is inserted into the borehole and the void between pipe and hole backfilled with a special grout solution so that the pipe is in close contact with the rock strata or earth. Fluid (referred to as ‘brine’ is then circulated through this loop and is heated up by the bedrock. Different rock types will give different results. In some cases, a number of boreholes will be made (for example, over a car park) to provide sufficient energy for the heat pump supply. If the ground is not suitable, horizontal loops can be laid or even trench filled ‘slinky’ loops, which are very simple to install. However, trench filled systems and horizontal systems require much more ground than vertical systems. If one has a pond or lake nearby then can lay a closed loop at the base of the pond (it needs to be about 2 metres deep), or simply extract the water directly out of the lake at low level and re-distribute it elsewhere in the lake.

Heat pumps can be cheaper to operate than other heating systems because, by tapping into free heat in the outdoor air, ground or water supply, they give back more energy-in the form of heat-than the equivalent amount of electrical energy they consume.

For example, in a heating mode, a highly efficient heat pump could extract energy from the earth and transfer it into a building. For every 1 KWh of electrical energy used to drive the heat pump, around 3 to 4 kWh of thermal energy will be produced. In a cooling mode, the heat pump works in reverse and heat can be extracted from a building and dissipated into the earth. Heat pumps which work in a heating mode are given a ‘coefficient of performance’ or ‘COP’ calculated by dividing the input kWh into the output kWh. This will give a COP figure, which varies with the input temperature and is the ratio of energy into energy out. In a cooling mode, the ratio is called the ‘energy efficiency ratio’ or ‘EER’. The higher the EER and COP ratios are, the more efficient the unit will be. Geothermal/GSHPs are self-contained systems. The heat pump unit is housed entirely within the building and connected to the outside-buried ground loop.

Conventional heating or cooling systems require energy from limited resources, e.g., electricity and natural gas, which have become increasingly more expensive and are at times subjects to shortages. Much attention has been given to sources that exist naturally and can be exploited as a natural phenomenon or what is known as renewable energy. Such energy includes geothermal energy, solar energy, tidal energy, and wind generated energy.

While all of these energy sources have advantages and disadvantages, geothermal energy, i.e., energy derived from the earth or ground, has been considered by many as the most reliable, readily available, and most easily tapped of the natural phenomena.

This study has dealt with the modelling of vertical closed-loop and ground source heat pump system. The challenges associated with the design of these systems originate from the fact that they present a unique type of heat transfer problem. First, there are inherent inabilities to make direct observations in the subsurface environment with respect to both space and time. Second, heat transfer within the subsurface environment can be highly transient. Consequently, a considerable amount of research in the past decade has been geared towards optimising the design and performance of GSHP systems and this study is part of those efforts.

The installation and operation of a geothermal system may be affected by various factors. These include, but are not limited to, the field size, the hydrology of the site the thermal conductivity and thermal diffusivity of the rock formation, the number of wells, the distribution pattern of the wells, the drilled depth of each well, and the building load profiles.

The performance of the heat pump system could also be improved by eliminating unnecessary running of the integral distribution pump. This would improve both the economic and the environmental performance of the system.

The results of soil properties investigation have also demonstrated that the moisture content of the soil has a significant effect on its thermal properties. When water replaces the air between particles it reduces the contact resistance. Consequently, the thermal conductivity varied from 0.25 W/m/K for dry soil to 2.5 W/m/K for wet soil. However, the thermal conductivity was relatively constant above a specific moisture threshold. In fact, where the water table is high and cooling loads are moderate, the moisture content is unlikely to drop below the critical level. In Nottingham, where the present study was conducted, soils are likely to be damp for much of the time. Hence, thermal instability is unlikely to be a problem. Nevertheless, when heat is extracted, there will be a migration of moisture by diffusion towards the heat exchanger and hence the thermal conductivity will increase.

Conclusions

Conventional heating or cooling systems require energy from limited resources, e.g., electricity and natural gas, which have become increasingly more expensive and are at times subjects to shortages. Much attention has been given to sources subject to sources of energy that exist as natural phenomena. Such energy includes geothermal energy, solar energy, tidal energy, and wind generated energy. While all of these energy sources have advantages and disadvantages, geothermal energy, i.e., energy derived from the earth or ground, has been considered by many as the most reliable, readily available, and most easily tapped of the natural phenomena.

This study has dealt with the modelling of vertical closed-loop and ground source heat pump system. The challenges associated with the design of these systems originate from the fact that they present a unique type of heat transfer problem. First, there are inherent inabilities to make direct observations in the subsurface environment with respect to both space and time. Second, heat transfer within the subsurface environment can be highly transient. Consequently, a considerable amount of research in the past decade has been geared towards optimising the design and performance of GSHP systems and this study is part of those efforts.

The installation and operation of a geothermal system may be affected by various factors. These factors include, but are not limited to, the field size, the hydrology of the site the thermal conductivity and thermal diffusivity of the rock formation, the number of wells, the distribution pattern of the wells, the drilled depth of each well, and the building load profiles. The performance of the heat pump system could also be improved by eliminating unnecessary running of the integral distribution pump. This would improve both the economics and the environmental performance of the system.

The results of soil properties investigation have also demonstrated that the moisture content of the soil has a significant effect on its thermal properties. When water replaces the air between particles it reduces the contact resistance. Consequently, the thermal conductivity varied from 0.25 W/m/K for dry soil to 2.5 W/m/K for wet soil. However, the thermal conductivity was relatively constant above a specific moisture threshold. In fact, where the water table is high and cooling loads are moderate, the moisture content is unlikely to drop below the critical level. In Nottingham, where the present study was conducted, soils are likely to be damp for much of the time. Hence, thermal instability is unlikely to be a problem. Nevertheless, when heat is extracted, there will be a migration of moisture by diffusion towards the heat exchanger and hence the thermal conductivity will increase.