Comparison of Above and Below Ground Seasonal Heat Storage

Nicholas Mooney

MECH4501

Supervised by Assoc. Prof. Kamel Hooman

Faculty of Engineering, Architecture and Information Technology

Abstract

Solar Photovoltaic and thermal panels are an excellent means of harnessing renewable energy and reducing Greenhouse Gas emissions, however, a number of flaws and high initial cost have prevented consumer uptake. This has been especially apparent in colder climates where gas and electricity heaters are used for space heating. In Europe, over 2/3 of household energy consumption is heating. With the addition of heating and heat storage elements, solar power can be an effective tool to combat heating costs. Towns like Rostock in Germany have demonstrated this with efficient district wide heating systems, powered primarily by solar energy and boosted with traditional boilers.

A similar system has been proposed as an option for use in Krakow, Poland, where solar energy would be stored seasonally underground on a per building basis. Excavation costs however are significant, so the primary goal of this thesis is to ascertain the feasibility and potential efficiency deficits of an above ground thermal storage tank.

Research was conducted into a range of aspects of this system to identify the best components to model and compare the systems’ performance. From this, a Python script was developed, capable of simulating system performance from one hour up to one year, under typical Polish weather conditions.

In all simulations, it was found that the above ground tank actually outperformed the underground one. While this is a pleasing result, further research is still recommended due to modelling constraints, which favoured the performance of the above ground storage. Specifically, a finite element analysis of the underground system should be conducted to assess the impact of the thermal memory of the surrounding soil.

Contents 1.0 Introduction ...... 1

1.1 Research Motivation ...... 1

1.2 Background of Existing Technologies ...... 1

1.3 Objectives ...... 3

1.4 Scope ...... 3

1.4.1 In Scope ...... 3

1.4.2 Out of Scope ...... 4

1.5 Approach ...... 4

2.0 Literature Review ...... 5

2.1 Solar Radiation as a Renewable Energy Source ...... 5

2.2 Solar Photovoltaics and Panel Cooling ...... 8

2.3 Solar Thermal Collectors ...... 10

2.4 Solar and Ground Source Space Heating ...... 12

2.5 Thermal Energy Storage (TES)...... 16

2.6 Heat Transfer Models...... 19

3.0 Method ...... 21

4.0 Validation ...... 25

4.1 Model Limits ...... 25

4.2 Comparison of Results to Literature ...... 26

5.0 Results ...... 29

5.1 AGTES ...... 29

5.2 UTES ...... 30

5.3 Combined Plots ...... 31

6.0 Discussion ...... 32

6.1 Constraints on 1D Steady State model ...... 32

6.2 The Role of Forced Convection ...... 32

6.3 The Weak Effect of the Heat Addition ...... 32

7.0 Conclusions and Recommendations ...... 33 8.0 References ...... 34

A.0 Appendices ...... 39

A.1 Equations ...... 39

A.2 Modification to Solar PV Panel for cooling...... 39

...... 39

A.3 Sample Weather Data ...... 40

A.4 Python Simulation Code ...... 41

Figure 1: Simplified schematic of the proposed system ...... 3 Figure 2: (a) Reduced energy density at different latitudes; (b) Altered solar irradiation dependent on season due to tilt in the Earth’s axis (Chiras, 2016) ...... 5 Figure 3: Relationship of monthly energy consumption vs monthly average temperature in Shanghai China (Yi-Ling, Hai-Zhen, Guang-Tao, & Jun, 2014) ...... 6 Figure 4: (a) Daily solar radiation on a vertical south-facing surface over a one year period; (b) Daily solar radiation on a vertical west-facing surface over a one year period (Chen, 2011) ...... 7 Figure 5: Percentage of households in four different Chinese cities using space heating at different times across a typical winter’s day ...... 7 Figure 6:(a) electrons rushing from the n- to p-layer; (b) formation of one way junction; (c) electrons freed by sunlight via the photoelectric effect forced across the p-n junction; (d) with metal contacts in place the PV circuit is complete (Chiras, 2016) ...... 8 Figure 7: PV panel cooled by 1-2L/min of water, flowing from the top to the bottom. (Raval, Maiti, & Mittal, 2014) ...... 10 Figure 8: A simple solar collector consisting of a copper plate beneath a glass pane, surrounded by insulation. A working fluid would be circulated through copper pipes soldered to the rear of the plate to move heat...... 11 Figure 9: Likely radiation wavelengths emitted by a solar collector compared to the solar spectrum; note that little of the solar spectrum exists above the 2휇푚 wavelength marker. (Chen, 2011) ...... 11 Figure 10: A modern evacuated tube solar collector (Chen, 2011) ...... 12 Figure 11: Typical reversible combined hot water and space heating heat pump (Lund, Sanner, Rybach, Curtis, & Hellström, 2004) ...... 13 Figure 12: Four of the most common methods to access ground source heat ...... 14 Figure 13: A typical active hydronic solar space heating system (Sarbu & Sebarchievici, Chapter 5 - Solar Water and Space-Heating Systems, 2017) ...... 15 Figure 14: Trombe wall passive heating (left) and cooling (right) system (He, Zhang, & Zhang, 2019) ...... 15 Figure 15: Implemented heat storage system in Friedrichschafen, Germany (Bodmann, et al., 2005) 17 Figure 16: Heating network employed in Rostock, Germany, combines both above and below ground TES (Bauer, et al., 2010)...... 18 Figure 17: The two potential configurations for TES in the proposed system ...... 21 Figure 18: Comparison of 200 and 20 Second Time Steps Over 5 Days ...... 25 Figure 19: Comparison of short term (5 day) and long term(1 year) simulations with small and large time steps, ...... 26 Figure 20: Comparison of heat addition method with ordinary resistance network...... 27 Figure 21:Comparison of the temperature of solar collector and PV panels over model year June 2014 to June 2015 ...... 28 Figure 22: One year UTES tank simulation, June to June ...... 28 Figure 23: Temperature of above ground tank, simulated over one year ...... 29 Figure 24: One year UTES tank simulation, June to June ...... 30 Figure 25: One year UTES tank simulation, June to June, with heat addition ...... 30 Figure 26: Both Tanks Simulated with no heat addition, with weather data from June 2014-June 2015 ...... 31 Figure 27: Comparison of UTES with and without heat addition and AGTES. Simulated with weather data taken from Krakow between June 2014 and June 2015 ...... 31

Table 1: Ground temperature used by month ...... 27

Nomenclature 휎 – Stefan–Boltzmann constant COP – Coefficient of Performance cp – Specific heat capacity, J/kgK Subscript h – heat transfer coefficient, convection, a – ambient air W/m2K abs – Solar collector absorber k – Thermal conductivity, W/mK air – Air L – Length, m b – Building m – Mass, kg e – Electrical conversion 푚̇ – Mass flowrate, kg/s g – Glass Q – Energy Flux, W gr – Ground q – Solar radiation, W/m2 gw – Propylene glycol-water mix R – Thermal resistance H – at outlet of heat pump (hot side) r – Radius, m in – inlet of PV and SC panels SA – Surface Area, m2 ins – thermal insulator T – Temperature, oC o – at outlet of building floor heating loop T – Ideal building temperature, oC set out – outlet of PV or SC panels t – Time, s pv – Photovoltaic panel UV – Thermal transmittance, W/K s – Sky V – Volume, m3 sc – Solar thermal collector

si – Silicon Greek t –area of contact to glycol mixture 휀 – Infrared emissivity th – Thermal 3 ρ – Density, kg/m tk – Tank 휂 – Efficiency w – Water 휏 – solar radiation transmittance

1.0 Introduction

1.1 Research Motivation Global Warming is one of the most significant threats to the future of modern society, with effects including – but not limited to – an increase in severity and frequency of extreme weather patterns, such as flooding, cyclones and drought; shortages of fuels and power; destruction of coastal property and infrastructure due to rising sea levels; and increased exposure to waterborne and food borne diseases, as well as disease carrying pests and insects (Reidmiller, et al., 2018). These effects have already begun occurring or are likely to occur in the near future (before 2050) primarily as a result of human activities since the industrial revolution (circa. 1850); namely the continued combustion of fossil fuels, production of greenhouse gases and destruction of the Ozone Layer by man-made TFCs (Tetrafluorocarbons) (Masson-Delmotte, et al., 2018). Thus, reduction of these emissions is crucial to the slowing and/or prevention of the effects of global warming. The energy supply sector alone (defined in Error! Reference source not found.) contributes over 35% of anthropogenic greenhouse gas emissions (Edenhofer, et al., 2014). Furthermore, in areas with cold climates like Europe, Canada and the US heating is essential and impacts the energy supply sector disproportionately when compared to more temperate areas like the Asia-Pacific regions. Between 2005 and 2016 space heating in European households accounted for around two-thirds of the continent’s overall energy consumption (Mourelatou, et al., 2018).

The energy supply sector – and in particular space heating – is an area of focus in preventing the effects of global warming by reducing greenhouse gas emissions. However, there has been a delayed uptake in technologies targeting this area due to high capital costs and several flaws that exacerbate other costs when compared to established heating and power generation technologies. In the UK only approximately 0.2% of energy supplied to households was generated onsite (Balcomb, Rigby, & Azapagica, 2014). While between 2005 and 2016 household electricity consumption has only reduced by 5% in Europe and in some countries – including Poland, where this study is based – electricity consumption has actually increased (Mourelatou, et al., 2018). This is in spite of the existence of a range of technologies that reduce or even negate the necessity of electricity consumption for major consumption sectors, including space heating. Designing and implementing efficient and effective energy generation and space heating solutions is crucial to reducing greenhouse gas emissions by reducing cost and incentivising with clear financial returns.

1.2 Background of Existing Technologies Solar radiation is a source of renewable energy capable of producing not only electricity, but also directly producing heat, unlike other commercially viable renewable energy sources like wind or hydropower. However, solar photovoltaic (PV) panels only convert approximately 5-17% of incoming radiation energy into usable electricity (Raval, Maiti, & Mittal, 2014). Unlike wind and hydropower

1 systems which have respective efficiencies in the order of 50% (Center for Sustainable Systems, University of Michigan, 2019) and 90% (The Clean Energy Coucil, 2012). Compounding this, under normal operating conditions, solar panels retain energy not converted to electricity in the form of heat, which further reduces their efficiency. At approximately 25oC, for every oC rise in temperature, PV panels experiences a 0.5% drop in efficiency; if a solar panel is cooled peak power can be increased by up to 20% (Raval, Maiti, & Mittal, 2014). Another issue with solar energy is the intermittence of power production. Fossil fuel reliant systems, hydro and wind power can all produce energy – and thus heat – during the night when it is often most needed, but PV panels stop producing energy well before then. Solar irradiation is also reduced during the winter months, further diminishing the effectiveness of solar systems. Solar thermal systems – which forgo electricity production to specifically harvest heat energy – have a far better efficiency – in the order of 80%) (Nshimyumuremyi & Junqi, 2019). When paired with a form of energy storage, they can effectively mitigate several of the issues with PV panels. However, the lack of electricity production limits the versatility of these systems.

Ultimately, it is a combination of both solar thermal and PV technology that most effectively harvests solar radiation and the pursuit of an effective combination of the two is the motivation for this research. (Hooman & Oclon, 2020) has proposed a combined solar thermal, cooled solar PV and seasonal thermal energy storage system which specifically targets the:

- low efficiency of solar PV Panels by cooling them and capturing and storing the excess heat - intermittency of solar energy by storing heat seasonally in a large thermal reservoir - lack of versatility of solar thermal systems by combining with solar PV panels - space heating crisis in Europe by producing and storing renewable energy for heating

As part of a larger project, this thesis will focus specifically on the comparison of two potential thermal storage techniques. Ideally an underground thermal reservoir (water tank) would be constructed to take advantage of the surrounding earth’s thermal memory – effectively increasing the size of the reservoir. However, excavation is both costly and logistically difficult in built up areas and thus is not always practical. Building an above ground insulated tank may be an effective way to reduce project cost and complexity, but still achieve an effective heat storage. The basic model for the system will follow recommendations and prior research by (Hooman & Oclon, 2020), with adjustments made based on findings made during the research process. A basic schematic of the system can be viewed in Figure 1.

Figure 1: Simplified schematic of the proposed system 1.3 Objectives The primary goal of this investigation was to realistically model the proposed system under likely weather conditions and identify the performance gap between the above-ground and below-ground storage tanks. The secondary goals were:

- To identify the approximate monetary cost gap between the two solutions - To rationalise a recommendation of storage technology

1.4 Scope The following identifies the extent to which a range of factors will be explored.

1.4.1 In Scope - This investigation is based in Krakow, Poland and, as such, all weather and ground temperature data will be based on historical data from this region. - The proposed solar PV, solar thermal and thermal storage system must be modelled realistically within the constraints of a 1D heat transfer network and will be validated by comparison to similar existing technologies. - The model will produce results containing the tank temperature data, energy loss totals (from tanks by conduction or convection), energy consumed by space heating and percentage of necessary heating energy produced by system.

- Modelling periods will range between one day and one season. - Steady heat transfer between systems. Non-steady heat captured by the ground and returned to the tank will be accounted for by increasing ground temperature to increase thermal resistance.

1.4.2 Out of Scope - System performance under weather conditions outside of Krakow. Regions of a similar climate may be evaluated by proxy, but performance in warmer or colder climates will not be evaluated - Modelling of technologies outside of those proposed by (Hooman & Oclon, 2020); comparisons to other technologies will be conducted by comparison of data. - System-wide analysis by Computational Fluid Dynamics (CFD) or Finite Element Analysis (FEA). Only heat resistance networks will be utilised in the global model due to system complexity. - Modelling of heat transfer within tanks – water is assumed to be well mixed and as such the system boundary will be at the inner tank wall. - Forced convection heat losses – it is assumed most components will be shielded from wind; hence only natural convection will be considered. - In-depth techno-economic analysis of system performance – only the cost difference between storage technologies will be considered.

1.5 Approach The approach taken to this investigation is as follows:

1. Identify simple mathematical models for heat gain and heat loss of individual components. 2. Combine component models to find a heat energy balance of entire system. 3. Gather and process weather data from Krakow; generate weather models representing expected weather for all seasons across a year, including best- and worst-case scenarios. 4. Run model; compare and validate results against similar existing systems. 5. Compare results of above- and below-ground tanks and recommend most effective solution after accounting for approximate cost difference.

2.0 Literature Review Section 2.0 presents an in-depth review of existing technologies and models pertinent to the proposed combined system, beginning with Solar energy and It’s role as a resource as well as investigating existing technologies that make up the system used for this thesis. This is followed by an investigation of existing combined TES systems and how they compare to the proposed system. Finally, a review of relevant heat transfer methods is presented in Section 2.6, justifying the choice of equations for the model.

2.1 Solar Radiation as a Renewable Energy Source Solar radiation is – by far – the most abundant naturally occurring energy source available to humans; estimates place total solar irradiation energy impacting the Earth yearly to be in the order of 5.46 × 106퐸퐽, where 1퐸퐽 = 1 × 1018퐽. While a large portion of this energy is absorbed by water, air and clouds or is reflected back into space, global energy consumption is only in the order of 500EJ, even 0.1% of yearly irradiation would be necessary to power the planet (Chen, 2011). In fact, yearly solar irradiation is so vast that a mere 1% of it could outpower the Earth’s entire coal reserve of approximately 480 gigatons (Gt) (Miller, 2004) which burns to produce energy at 24 Megajoules per Kilogram (Mah, et al., 2018).

(a) (b) Winter solstice Northern hemisphere tilted o 23.5 away from the Sun

Spring and autumn equinoxes

Summer solstice Northern Hemisphere tilted o 23.5 toward the Sun

Figure 2: (a) Reduced energy density at different latitudes; (b) Altered solar irradiation dependent on season due to tilt in the Earth’s axis (Chiras, 2016)

With such remarkable figures it would be remiss not to take advantage of solar energy; however, it is with good reason that solar power is not the primary means of energy production today. First and foremost is the intermittency of solar radiation due to the rotation of the Earth. Energy is required 24 hours per day, 365 days per year, but solar energy is only available during the day and the energy that does reach the Earth’s surface is reduced by a myriad of factors. Depending on its latitudinal location, a particular surface will experience reduced irradiation according to its distance from the equator, demonstrated simply in Figure 2(a) (Chiras, 2016). Irradiation per unit area perpendicular to the Sun’s rays is approximately constant but upon impacting the Earth’s surface, it is spread further north and south of the equator due to Earth’s spherical shape. This effect is exacerbated dependent on the season due to the tilt in the Earth’s surface, as seen in Figure 2(b) (Chiras, 2016); resulting in less radiation density and shorter days or shorter radiation periods. This is crucial to the success – or lack thereof – of solar radiation as an energy source; the available amount of energy is inversely related to the energy demand during winter (due to heating demand) but proportional to energy demand in summer (Figure 3 (Yi-Ling, Hai-Zhen, Guang-Tao, & Jun, 2014)). This effect is skewed further away from the spike in energy demand in summer in cold climates with a higher spike in winter; Figure 3 is based in Shanghai, China, which is both closer to the equator and warmer than Krakow, where this thesis is based.

Figure 3: Relationship of monthly energy consumption vs monthly average temperature in Shanghai China (Yi-Ling, Hai- Zhen, Guang-Tao, & Jun, 2014)

(Chen, 2011) points out that the orientation of a surface also plays a role in its daily irradiation across different seasons; Figure 4 demonstrates the difference between a south-facing surface and a west- facing one. Of particular note is that irradiation peaks during winter on a south facing surface (Figure 4(a)) but peaks during summer on a west-facing surface (Figure 4(b)) and has lower overall radiation. We also see variation – as above – based on latitude 휙, however it behaves inversely to how we might expect as the surfaces assessed are vertical to the Earth’s surface, not parallel. The optimum stationary

6 position for a particular surface is south facing and tilted equal to it’s latitude, but more radiation can be captured if the surface follows the apparent motion of the sun (Chen, 2011).

(a) (b)

Figure 4: (a) Daily solar radiation on a vertical south-facing surface over a one year period; (b) Daily solar radiation on a vertical west-facing surface over a one year period (Chen, 2011)

Similarly to seasonal energy consumption changes, daily energy consumption is often poorly timed to take advantage of peak solar irradiation. (Hu, Yoshino, & Jiang, 2013) investigated the number households implementing space heaters in four different cities and at which time they were employed. Peak solar irradiation usually occurs within one to two hours of midday, but peak energy usage begins nearer to 4pm; it is clear that solar energy cannot directly be used to cover heating needs (Figure 5). Indeed, at peak energy consumption (~8pm), there would 0 solar irradiation. Applying these results to a colder climate like in Krakow, the effects would only be intensified, with higher peak energy usage and less daily irradiation to draw from. For a solar energy system to be successful in such climates it must be both efficient and able to store harvested energy for later use; ideally, energy harvested during summer would be utilised during winter.

Figure 5: Percentage of households in four different Chinese cities using space heating at different times across a typical winter’s day

2.2 Solar Photovoltaics and Panel Cooling Solar PV panels harvest solar radiation, converting the energy of photons to electricity via the photoelectric effect. When an incoming photon collides with an atom’s surrounding electron cloud it will impart its energy if, and only if, it has energy greater than a material dependent threshold value. A valence electron (the most energetic electrons in an atom) will then be ejected from the cloud with kinetic energy equal to the difference between the photon’s energy and the material’s threshold value. Semiconductors are materials that only allow movement of electrons (current) when the photoelectric effect is in play (or when temperatures are sufficiently high). This allows the flow of electrons to be controlled by “doping” – introducing small impurities into a semiconductor’s lattice structure – such that a voltage is induced. A solar PV panel is created by laying a thin, slightly negatively charged “n- layer” that is doped with an element with excess electrons (like phosphorous) on top of a thicker “p- layer”, doped with an element containing a slight deficit of electrons (like boron). At the junction between the two layers, electrons rush across the border from the phosphorous, filling the gaps in the boron bonds with surrounding silicon. This results in a region of positivity at the bottom of the n-layer and a region of negativity at the top of the p-layer; essentially a one way gate that only allows electrons to reach the “holes” in the boron at the bottom of the p-layer by completing a circuit. When sunlight impacts these layers, the photoelectric effect comes into play and electrons are freed but forced to move in one direction only, which creates useable electricity (Chen, 2011).

(a) (c)

(b) (d)

Load

Figure 6:(a) electrons rushing from the n- to p-layer; (b) formation of one way junction; (c) electrons freed by sunlight via the photoelectric effect forced across the p-n junction; (d) with metal contacts in place the PV circuit is complete (Chiras, 2016)

As previously mentioned, the other way to free electrons in a semiconductor is heat, as such, it is easy to misinterpret rising temperature within a panel as something that would increase current. However, the opposite is true; the rising temperature indiscriminately frees electrons to flow throughout the entire lattice and so the one-way p-n junction breaks down meaning current flow is not uni-directional

8 and little to no electricity is generated. This is why solar panels have reduced efficiency (percentage of incoming photon-energy converted to flowing electrons) on hot days; the higher the temperature the worse the breakdown of the p-n junction and less electrons complete the circuit through the desired load.

The “band gap” (the previously mentioned energy threshold value) is the energy required from the photoelectric effect to force an electron from the “valence band” (an atom’s outer electron shell) to the “conduction band” (where the electron is free to flow). One may suspect it is advantageous to choose a semiconductor with a low band gap, such that the majority of incoming photons are able to liberate electrons, but this results in most photons having more than the threshold energy. The excess energy is converted to kinetic energy as mentioned in paragraph one, which is then dissipated as heat. This results in the panel not only losing efficiency due to temperature rise but also losing efficiency as less of the solar spectrums energy is available for conversion.

To combat the typically low efficiency of solar panels there are two viable options. Multi-junction cells contain multiple thin layers of different semiconductors, each with different band gaps, allowing them to take advantage of a wider range of frequencies within the solar spectrum. While they can reach efficiencies of up to 40% (Chiras, 2016), they are extremely expensive. The second option is to cool the panels such that the cells always run closer to maximum efficiency; this is usually done by passing a fluid over the surface or the rear of the panel. While cooled panels still only target a narrow section of the solar spectrum, the efficiency boost is significant when compared to ordinary PV panels, especially in summer and hot climates. There air two main types of panel cooling.

Air Cooled Panels Air cooled panels may be split into two categories, forced and passive. Passive air coolers use a metal heat sink mounted to the rear of the panel, often with fins to increase exposure to air; they rely on natural convection. Maximum power can be raised by as much as 7.5% and panel temperatures reduced by 10oC by simply attaching 3cm angled fins (Popovici, Hudişteanu, Mateescu, & Cherecheş, 2016). Further improvements can be made by skimming power directly from the panel to run an active cooling system. (Farhana, Irwan, Azimmi, Gomesh, & Gomesh, 2012) found a 12oC drop in panel temperature with their system, while electrical efficiency was boosted from ~7% to ~9.5% in a combined PV/Thermal system by the addition of two fans (Ameri, Mahmoudabadi, & Shahsavar, 2012). Despite the significant improvements in air cooled panels, the heat extracted is difficult to utilise and almost impossible to store. For this, a liquid cooled panel is necessary.

Liquid Cooled Panels Tests with cooled panels have been conducted (Figure 7) which saw the peak power production increasing by as much as 20% and overall efficiency reaching 40% from 6% after taking into account energy captured by the water (Raval, Maiti, & Mittal, 2014). (Zubeer, Mohammed, & Ilkan, 2017)

9 completed a review of relevant panel cooling techniques, making note of slightly lower but still similar efficiency gains (13% from 7.8% in one setup and temperature reductions of 20% accompanied by efficiency gains of 9% in another). However, both of these systems utilised heat exchangers running over the rear of the panel, and as (Nižetić, Čoko, Yadav, & Grubišić-Čabo, 2016) point out in their study, cooling the front side is more effective than cooling the back. The refractive index of the liquid passing over the panel also assists in gathering energy, explaining why the panel in Figure 7 outperforms others investigated. A front side cooling method similar to this will be assumed for the panels modelled in this thesis.

Figure 7: PV panel cooled by 1-2L/min of water, flowing from the top to the bottom. (Raval, Maiti, & Mittal, 2014) 2.3 Solar Thermal Collectors A solar thermal collector (STC) functions much more simply than a PV panel, simply collecting and storing heat, rather than converting it to electricity; as a result, STC panels operate with much higher efficiency than PV panels. A wide range of STCs exist, as such, the core commercial offerings will be investigated here. Efficiencies of STC panels are not as simple to evaluate as PV panels as they vary greatly, depending on the difference between the inlet and ambient or panel and ambient temperatures. Efficiency estimates will generally come as a range relevant to the expected operating temperatures.

Flat Plate Collectors (FPCs) The oldest solar thermal technology still in regular use today, flat plate collectors have been successfully implemented since the 1920’s. Modern FPCs are relatively cheap to manufacture, achieve respectable efficiencies in the region of 40-60% (Kalogirou S. A., 2009) and are extremely durable – thousands of units built in the 1920’s in Florida are still in operation today (Chen, 2011). As illustrated in Figure 8, a simple FPC consists of an insulated box containing a copper plate beneath a glass pane, to transport heat energy a working fluid would be circulated through copper pipes soldered to the rear of the plate. The working fluid is usually then passed through a heat exchanger to a heat storage tank.

Incoming solar radiation Glass Ambient

Air

Copper plate Copper pipe

Figure 8: A simple solar collector consisting of a copper plate beneath a glass pane, surrounded by insulation. A working fluid would be circulated through copper pipes soldered to the rear of the plate to move heat.

Modern FPCs also use a coating to capture more energy and specialised glazing to trap that energy. By making the collector a near-blackbody with a matte black coating it will absorb almost all radiation, but also emit a significant amount. Due to the plate’s low temperatures compared to the sun, solar collectors radiate energy at a much longer (lower energy) wavelength. The collector is enclosed by a glass front panel manufactured to reflect or block long wavelengths but permit higher energy radiation from the sun to pass through, thus energy reemitted by the collector is effectively trapped inside (Kalogirou S. A., 2009). This principle is demonstrated graphically in Figure 9. Radiant heat losses are effectively reduced in FPCs, but they are still susceptible to natural and forced convection, from the air inside the panel conducted by the glass and then away by the ambient air or wind.

Figure 9: Likely radiation wavelengths emitted by a solar collector compared to the solar spectrum; note that little of the solar spectrum exists above the 2휇푚 wavelength marker. (Chen, 2011)

The cooled PV panels proposed for use in this thesis’ simulation by (Hooman & Oclon, 2020) would function very similarly to an FPC, simply replacing the collector plate with a PV panel and running front side cooling similar to Figure 7.

Evacuated Tube Solar Collectors (ETSCs) These collectors build on the concepts of flat plate collectors, being a heat exchanger with an absorbent coating surrounded by specialised glass but differ dramatically in shape. As seen in Figure 10, ETSCs are a pair of concentric glass tubes, with the outer surface of the inner tube covered in a selective absorption coating, a spacer at one end for support and an opening at the other for the working fluid. The space between the two tubes is evacuated to approximately 10−4pa (Chen, 2011), greatly reducing convective losses; this results in an efficiency less dependent on ambient temperature and much higher efficiency in cold climates, when compared to a flat plate solar collector (Barone, Buonomano, Forzano, & Palombo, 2019). To create an effective solar collector the evacuated tubes are arranged side by side to create a panel and the working fluid networked to a manifold containing the heating load (usually water).

Figure 10: A modern evacuated tube solar collector (Chen, 2011)

Used in this thesis, ETSCs were selected for their comparative advantages over FPCs in cold climates.

2.4 Solar and Ground Source Space Heating Space heating and cooling by a renewable energy source, like solar power has been a practice used for thousands of years; by the simple orientation of windows and walls heat collection can be reduced or increased. Even organisms as simple as termites build their nests oriented east-west to control temperature during hot Australian summers (Monteith, 2011). However, to ensure fine temperature control of buildings in harsh climates, it is necessary to harness natural resources and combine them with heat pumps. The following is a review of common space heating methods using solar and ground heat sources.

Ground Source Heating At a certain depth underground, the temperature will remain almost constant all year-round, at approximately the annual average solar temperature of the surface (Derradji & Aiche, 2014). (Chen, 2011) suggests that temperature remains constant around 10m, though numerical simulations and measurements by (Kalogirou & Florides, 2004), indicate that the fluctuation of temperature is relatively small (2-4oC) over the course of a year at depths as small as 3m. Ultimately, ground temperature is a function of “incident solar radiation, rainfall, seasonal swings in overlying air

12 temperature, local vegetation cover, type of soil, and depth in the soil” (Ghahreman, Bazrafshan, & Gharekhani, 2010), so the depth of constant temperature is entirely dependent on location. Limited temperature data exists specifically for Krakow, however models for Poland as a whole estimate that average ground temperature is in the order of 8-10oC (Kurpaska, 2010), (Chwieduk, 1996).

Constant ground temperature is crucial to the success of ground source heat pumps; during winter the ground temperature is above that of the surface temperature and during summer it is lower. Hence during summer, heat from buildings can be removed using the earth as a heat sink and during winter heat can be extracted from the ground. This process is generally completed with a reversible heat pump, using water as a working fluid to extract heat from the ground and a refrigerant as the working fluid to heat the building. A typical heating cycle can be observed in Figure 11.

Figure 11: Typical reversible combined hot water and space heating heat pump (Lund, Sanner, Rybach, Curtis, & Hellström, 2004)

Heat pumps are rated in efficiency by their coefficient of performance; that is, the amount of heat output, compared to the amount of energy input for compression (and hence heating). A typical heating cycle will have a coefficient of performance of around 3-4 (Lund, Sanner, Rybach, Curtis, & Hellström, 2004). Methods of accessing the available underground heat range depending on location and soil type; Figure 12 demonstrates the most common types. Open loop wells (Figure 12 (c) and (d)) are less suited to cold climates as ground water is the working fluid and is susceptible to low temperatures as it approaches the surface. Horizontal closed loops are similarly not suited to cold

13 climates as they operate close to the surface (~2m) (Lund, Sanner, Rybach, Curtis, & Hellström, 2004); however, they are generally cheaper to implement as excavation depth is reduced. Finally, vertical closed loops can operate using an antifreeze fluid, making them more suited to cold climates (Sarbu & Sebarchievici, General review of ground-source heat pump systems for heating and cooling of buildings, 2014). The system proposed for this thesis by (Hooman & Oclon, 2020) would use a shallow closed loop style heat collector to operate a heat pump, though the main heat source would be a tank, not simply the surrounding ground.

Figure 12: Four of the most common methods to access ground source heat

In a typical heating cycle, once heat is extracted from the source and passed to the internal working fluid, it is usually distributed to a hot water tank followed by a floor heating system. A system of pipes is distributed throughout or under the flooring and above a reflective surface. Heat is conducted and radiated from the pipes to the flooring then convected to the building’s air. This creates a homogenous room heating with lower temperatures (40-50oC) than other heating methods like radiators, which operate at higher temperatures (70-90oC) to compensate for only producing heat from a relatively small point (Maleh & Tine, 2011). Due to the relatively low incoming temperature of heat from renewable sources, it is assumed that the proposed building will have a radiant floor heating network.

Solar Heating Solar heating is often proposed in conjunction with ground source heat pumps due to their lack of inherent thermal energy storage; this combination has been proposed as early as 1956 (Sarbu & Sebarchievici, General review of ground-source heat pump systems for heating and cooling of buildings, 2014). However, this is not always the case, (Maleh & Tine, 2011) sufficiently heated a small room using 10m2 of solar collectors, a small auxiliary boiler and an above ground storage tank, while standalone hydronic (using water or antifreeze to collect solar energy) systems using above

14 ground storage are stated to supply up to 80% of heating needs (Sarbu & Sebarchievici, Chapter 5 - Solar Water and Space-Heating Systems, 2017).

Figure 13: A typical active hydronic solar space heating system (Sarbu & Sebarchievici, Chapter 5 - Solar Water and Space-Heating Systems, 2017)

Air based solar systems are also effective in certain climates, (Reid, 2001) found 70% of space heating needs were supplied in winter. These systems may be either active – pushing heated air with fans, as in the case of (Reid, 2001) – or passive, utilising natural convection currents to draw heat in or out of a room. A common passive heating system, utilised on walls with heavy sun exposure, called a Trombe Wall is displayed in Figure 14. Active air circulation systems function similarly but accelerate flow rates with strategically placed fans. Air circulation systems are, however, not compatible with the solar PV cooling goal of this project, as such a hydronic system must be used

Figure 14: Trombe wall passive heating (left) and cooling (right) system (He, Zhang, & Zhang, 2019)

The combination of both solar and ground source heat pumps (similar to this project) is a field already being explored and yielding positive results. By alternating the use of the two kinds of pumps, overnight load on the solar heat pump was able to be reduced and the COP was increased when

15 compared to a purely solar system (Bi, Guo, Zhang, & Chen, 2004). Districts in Denmark use used combined ground and solar source heat pumps to combat high electricity costs and carbon taxes, with over 50% of heating needs covered by district heating networks (Li, 2015).

2.5 Thermal Energy Storage (TES) TES is the crux of this project; by storing the heat gained in summer for use in winter less energy is wasted and better heating can be achieved. It is generally accepted that large scale above ground heat storage is less effective than below ground; however this disparity may be acceptable when the significant cost of excavation is taken into account. Below is a review of relevant thermal energy storage systems.

Underground Thermal Energy Storage (UTES) Underground Thermal Energy Storage is the practice of using a large thermal body with a high specific heat capacity to store thermal energy while it isn’t necessary, for later use when producing energy may be difficult or expensive. It is commonly used in conjunction with solar applications due to the daily intermittency of sunlight, excess of energy during summer and lack thereof in winter. The storage medium for a UTES, or indeed any kind of TES, is often water, due to its high specific heat capacity, availability and low cost. As soil has a thermal memory, if the body of water it contacts is heated for long enough, it will also begin to store energy. If a tank has its heat energy depleted quickly, the thermal memory of the soil can often return its energy to the system. However, the tank will generally need to be insulated from cold surface temperatures with some kind of ground covering (Novo, Bayon, Castro-Fresno, & Rodriguez-Hernandez, 2010).

Solar heating and UTES are implemented in block or district systems and can generally be divided into two categories. Short-term or diurnal storage generally supplies 10-20% of hot water demand while seasonal heat storage systems can supply more than half of the space heating and hot water demands (Schmidt, Mangold, & Müller-Steinhagen, 2004). A number of such seasonal systems exist throughout Denmark and Germany, usually as a combination of a number of heat sources, not simply solar. Copenhagen powers 98% of its municipal city are with a combination of renewable and non- renewable sources, including incinerators, gas burners and combined heat and power (CHP) plants. Rather than storing heat centrally, tanks and sources are distributed throughout the city to spread load and better deal with peak demand. Copenhagen is aiming to be the first carbon neutral capital city and as such has installed a test solar collector field complete with thermal storage tank as well as a geothermal CHP plant supplying 13MW of geothermal energy (Li, 2015). The Copenhagen model is pertinent to the thesis as the distribution of heat storage throughout the city is a potential implementation of the proposed system. Multiple buildings would be equipped with panel arrays and tanks and then networked together, decentralising heating and allowing heat to be optimally shared between buildings with different heat loads. With this infrastructure already in play the addition of

16 solar arrays to replace or subsidise existing heat sources is more enticing. An apartment complex in Lystrup, Denmark, has also demonstrated that high temperature district water heating is not necessary. By increasing system pressure and narrowing pipes, losses were reduced and space heating water was supplied to 40 apartments at 50oC without the need to reheat at any point (Schmidt, et al., 2017). This demonstrates that losses in heat transport can almost be neutralised even within small networks and between buildings.

Figure 15: Implemented heat storage system in Friedrichschafen, Germany (Bodmann, et al., 2005)

Beginning in 1996 and being delivered in three stages, a solar assisted district heating system was constructed in Friedrichschafen, Germany. Solar fractions of 21-30% were delivered during this time and with the assistance of a gas fired boiler, tank temperature were kept above 40oC with a peak of 85oC, attributed to the addition of more solar panels and summer heat 2004 (Bodmann, et al., 2005). A simplified schematic of the Friedrichschafen system is shown in Figure 15.

Finally the 2010 review “German central solar heating plants with seasonal heat storage” (Bauer, et al., 2010) states that the best solar fraction of the plants reviewed was 57% at the “Rostock CSHPSS” (Central Solar Heating Plants with Seasonal Storage). This was achieved using a buffer tank of around 30m3 volume, which was kept hot at all times and when unused was fed to a lower temperature aquifer TES (45oC)(Figure 16). The buffer tank bears a strong resemblance to the storage tank proposed for this project and its lower temperatures are similar to that of Lystrup; such a system being commercially implemented and working is promising for the feasibility of the thesis topic.

Above-Ground Thermal Energy Storage (AGTES) AGTES is generally seen as a less commercially viable option when compared with UTES, due to the difficulty of building large-scale, insulated thermal storage containers. On a small-scale, however, the

17 technology is very similar to that which would be found in household gas boilers or electric hot water systems. The water is heated in advance and stored for use in the short-term. These systems, however, don’t have sufficient thermal mass to store heat energy on a seasonal scale and larger and outdoor tanks must be considered.

The Zineg Project is one example of a larger scale above ground heat storage system. By storing solar energy gained both directly from the sun and also from latent heat in dew that condensed on roof mounted fins, energy consumption in heating was reduced by up to 81% (Schuch, Dannehl, Miranda- Trujillo, Rocksch, & Schmidt, 2014). While this approach isn’t directly applicable to the originally anticipated residential distribution, there is no reason why the proposed system couldn’t be applied to a greenhouse and the combined heat and power aspect could assist in powering lighting and pumps to further reduce emissions.

The previously mentioned Rostock installation also used a semi-large scale AGTES as its buffer storage is an above ground tank and at 30m3 is around half the volume of the tank proposed for use in this thesis. The success of this project indicates that it may be advantageous to utilise AGTES on a per building level and UTES at a community scale; this would be economically advantageous as the cost of excavation would be reduced by distribution throughout a community.

Figure 16: Heating network employed in Rostock, Germany, combines both above and below ground TES (Bauer, et al., 2010).

The working fluid of a solar TES was mentioned earlier as usually being water, however this is not necessarily the ideal fluid for thermal energy storage; some papers suggest that a working fluid in the traditional sense may not be needed at all. (Essen, et al., 2009) were among a number of groups investigating the potential for MgSO4 as a thermochemical heat storage. The paper notes that the heat

18 energy provided is stored in chemical bonds through a reversible action, with only 10% sensible heat loss. MgSO4 is specifically mentioned as an option for small-scale applications, and is capable of storing up to 1GJ/m3, although it is noted that it needs to be stored at a higher pressure than water to work most effectively, which incurs cost (Ferchaud, Scherpenborg, Zondag, & Boer, 2014). While

MgSO4 and working fluids generally are not specifically to be investigated, the efficacy of this substance makes a compelling case for investigating combination energy and heating systems. If technology like this, that can miniaturise the storage tank and prevent the need for excavation were to gain traction commercially, small scale – and even large-scale – deployment of the proposed system becomes even more attractive. However as this is a future technology, without a current commercial outlet, it should be considered as a potential future replacement or improvement to the system’s tanks.

2.6 Heat Transfer Models A range of modelling methods and representative equations were reviewed as candidates for implementation in the heat transfer model. Simplicity was key, due to the long simulated time periods and number of components. The highlights of the reviewed content is presented herein.

Given that prior research and modelling had been done on this topic, the natural starting point was the model used by (Hooman & Oclon, 2020). The primary component of this model was an inhouse MATLAB script which ran a finite element analysis (FEA) of the ground and tank, designed to show the way in which the ground held heat. The model was overly complex for comparison of the two tanks and had no information on modelling the above ground tank; the solar energy equations were also unrealistic to implement as they didn’t take into account temperature difference between fluids and simply added heat energy to the tank. However, parts of the model were useful, including the overarching heat balance, and the simple heat pump model. By comparing these to concepts and examples in “Heat Transfer: A practical Approach” (Cengel, 2004), the formulas were corroborated. A more complex heat pump and building model could have been implemented like that of “Studying, Testing and simulating floor heating solar system” (Maleh & Tine, 2011), however, as the comparison of the tanks is the focus of this project only a moderately accurate heat load is necessary – provided it remains consistent between tests. The thermal transmittance figure of the building used by (Hooman & Oclon, 2020), was checked against values used by (Maleh & Tine, 2011) and is relatively close.

To apply the solar energy to the system, equations from “Physics of Solar Energy” (Chen, 2011) were considered. As this is a book targeted primarily at scientists, many of the equations were overly complex aiming to thoroughly cover all aspects of solar radiation, rather than provide an effective, if imperfect representation of solar energy. The book did, however, provide a second confirmation of the heat pump equation selected previously selected from (Cengel, 2004) and (Hooman & Oclon, 2020). A Computational Fluid Dynamics (CFD) approach – similar to that of (Raval, Maiti, & Mittal, 2014) – was considered but dismissed due to complexity and inability to simulate long periods of time (like

19 the planned one year simulation). Eventually the simplified approach by (Ndiaye, 2015) was selected; while it is complex and very sensitive to step time, once values and functions were entered it was simple to incorporate into the heat balance. The approach could also be easily adapted to the cooled PV panels.

Finally, the tank heat loss model. (Ndiaye, 2015) suggested a tank model by (Duffie & Beckman, 2013), however, it was oversimplified and was especially unsuited to calculations for the UTES. Keeping in mind that the key values were the heat energy stored and overall temperature fluctuation inside the tanks so again CFD and FEA approaches were over complicated; it was not necessary to visualise the heat loss – graphing the resultant numbers would be sufficient. Returning to “Heat Transfer: A practical Approach” (Cengel, 2004); a heat resistance network was selected to model the UTES and AGTES (conduction and convection respectively). All approaches considered were 1D, due to the long thin nature of the tank.

3.0 Method The proposed system to be simulated is shown as a simple schematic in Figure 17; the following section will discuss the equations and process used to simulate these two cases, across a variety of situations using Python.

Figure 17: The two potential configurations for TES in the proposed system

The key equation to the system is the heat balance, that governs energy losses and gains to and from the tanks. This differential equation could be simply solved with a linear numerical approximation with the system assumed to be steady state. It was chosen to align with the model used in prior work for this system.

푑푇 푚 푐 푡푘 = 푄 + 푄 − 푄 − 푄 푤 푝,푤 푑푡 푠푐 푝푣 푔푟/푎𝑖푟 ℎ푝 (1)

Equation one balances 푄𝑖푛 = 푄푠푐 + 푄푝푣 against 푄표푢푡 = 푄푔푟/푎𝑖푟 + 푄ℎ푝 to ascertain a temperature difference in the tank at hand.

Taken from (Hooman & Oclon, 2020) and compared and confirmed against (Chen, 2011), the heat pump acts according to:

1 푄 = 푄 (1 − ) ℎ푝 푏 퐶푂푃

(2)

Where Qb represents the building heat demand and COP is the Coefficient of Performance of the heat pump (equation 3). Measures were taken to prevent the heat pump from operating at temperatures below 10oC, with the assumption that during this downtime a gas boiler would take over heating, allowing the tank to recover.

푇퐻 푄푏 = 푈푉푏(푇푠푒푡 − 푇푎); 퐶푂푃 = 휂ℎ푝 푇퐻 − 푇표

(3)

Losses to the surrounding earth or air from the UTES and AGTES respectively have been characterised by a thermal resistance network (equation 4). It is assumed that the tanks are well mixed due to the three sets of heat exchangers running inside them, hence the control volume for temperature loss has been set at the inside of the tank shell leading outwards (dotted circles Figure 17). Expansions for Rtotal can be found in Section 0.

푇∞ − 푇푡푘 푄푔푟,푎𝑖푟 = 푅푡표푡푎푙

(4)

The equations used to assess the incoming solar energy are those described in (Ndiaye, 2015), stemming from 2 differential equations, relating temperature change in the glass and absorber panels to the outgoing fluid temperature to the tank. The equations were easily modified for use with the cooled PV panels by a simple change of variables and the subtraction of a Pout (Equation 7) variable according to equation blank from (Raval, Maiti, & Mittal, 2014). The cooled PV panels are essentially modified flat plate solar collectors (Figure 8), with the plate replaced by a PV panel as seen in Appendix A.3.

푑푇푔 푚 푐 = 푄 + 푄 − 푄 − 푄 푔 푝,푔 푑푡 1 2 3 4

(5)

푑푇 푚 푐 푎푏푠 = 푄 − 푄 − 푄 푎푏푠 푝,푎푏푠 푑푡 5 2 6

(6)

푑푇푝푣 푚 푐 = 푄 − 푄 − 푄 − 푃 ; 푃 = 휂 퐴 푞 푝푣 푝,푝푣 푑푡 5 2 6 표푢푡 표푢푡 푒 푝푣

(7)

Q’s 1 through 6 are the various heat losses and gains from the evacuated tube SCs and PV panels.

They contain as follows: Q1 – energy absorbed by outer glass, Q2 – energy radiated between the absorber and the outer glass, Q3 – heat radiated from the glass to the sky using Swinbank equation for

Ts (Swinbank, 1963), Q4 – heat convected from the glass to ambient air, Q5 – heat absorbed by SC absorber or PV panel and Q6 – heat carried away by glycol-water mixture.

푄1 = 훼푔퐴푔푞

(8)

4 4 푄2 = 퐴푎푏푠휎휀푎푏푠(푇푎푏푠 − 푇푔 )

(9)

4 4 1.5 푄3 = 퐴푔휎휀푔(푇푔 − 푇푠 ); 푇푠 = 0.0552푇푎

(10)

푄4 = 퐴푔ℎ푔푎(푇푔 − 푇푎)

(11)

푄5 = 휏푔훼푎푏푠퐴푎푏푠푞

(12)

ℎ푔푤̇ ,푎푏푠퐴푡 − 푚̇ 푔푤푐푝,푔푤 푄6 = 푚̇ 푔푤푐푝,푔푤 (1 − 푒 ) (푇푎푏푠 − 푇푖푛)

(13)

Finally, the outlet temperature of the panels can be calculated by Equation 14:

ℎ𝑔푤,푎푏푠퐴푡 − 푚̇ 𝑔푤푐푝,𝑔푤 푇2푓 = 푇푎푏푠 − (푇푎푏푠 − 푇𝑖푛)푒

(14)

The outlet temperature is critical to the temperature accuracy of the tank; previous iterations of the model simply added energy from the panels, but this led to rapid and excessive temperature growth. In reality, energy cannot simply be added to the system, a function of the temperature disparity between the two fluids must be used. To achieve this, it was assumed that – for a particular timestep –

23 the temperature of the outgoing glycol mix from the tank would be approximately equal to the temperature of the tank when it arrived as the mass of the tank is large and so temperature gain would be marginal. Hence Equation (15) was used to isolate the amount of solar energy input by the solar arrays.

푞 = 푚̇ 푔푤푐푝,푔푤(푇표푢푡 − 푇푡푘)

(15)

Disparities between the non-steady FEA methods and the thermal resistance network in use were identified in the case of UTES. Due to the inherent steady state assumption of the resistance network, no heat can be stored in the surrounding earth, and hence a significant portion of the heat seen to be retained in (Hooman & Oclon, 2020) is lost. The AGTES, however, is an excellent example of a steady state system; other than the thermal body of water there is no other place to store a significant amount of heat, so any energy not in the tank is almost certainly passed to the air by convection. This puts the UTES at a disadvantage, so to counter this a strategy was devised. The use of radiation formulas necessitates small timesteps, as the temperatures to the power of four can quickly grow out of hand if the estimate is overshot. This means that longer simulations are time consuming to complete. If the simulations are split into individual but semi-continuous months or seasons with initial conditions altered at the beginning of each, but factors like current tank temperature, absorber temperature, etc. kept constant, processing time would be greatly reduced and ground temperatures could be increased in warmer months according to energy “lost” through the insulating ground layer. This was implemented with an estimated 30% of heat passing through the ground becoming trapped near the tank and has been validated in section 4.0.

Weather The weather model used in the original tests of the UTES system (Hooman & Oclon, 2020), was a simplified model designed to demonstrate a principle. With this thesis, however, it was important to test the system with real weather data to verify that it could withstand natural fluctuations and cold spells. Weather data was purchased with free student credit from (Solcast, 2019), covering hourly solar radiation and air temperature data in Krakow from 2014-2018. By comparison, the previous data utilised seems generous. Originally, a script was written to parse the weather data of 5 years to create a likely to occur “average year” model. This was found to flatten out extreme or unusual weather patterns, which is disadvantageous in assessing model performance in real world conditions; ultimately, the average year model was scrapped and a year-long period from June 2014 to June 2015 was chosen to compare models. This period has both periods of strong radiation in the summer but also extremely low radiation periods in winter – some days failed to even reach 50W irradiance.

4.0 Validation Before presenting results, the model outputs must be validated against their own limits and against similar systems from literature.

4.1 Model Limits It was noted during model testing that the time step used to numerically solve the differential equations was capable of crashing the model during run if it was too large. Due to the significant simulation lengths (up to one year), it was computationally expensive and time consuming to run the model based on a small time step. Timesteps in the order of seconds seemed especially unnecessary as the weather data it relied on was only updated hourly. However, due to the radiation equations generating number in the order of 108 (Section 3.0, Equations 9 and 10), if the linear approximation overshot slightly, it was found to crash the model, as numbers too large for Python to handle were generated. Tests were conducted to ensure that timesteps used stayed within a reasonable value.

Comparison of 200 and 20 Second Time Steps Over 5 Days 40.000

35.000

30.000

25.000

20.000 200s 20s

15.000 Temperature (C) 10.000

5.000

0.000 5 day Period

Figure 18: Comparison of 200 and 20 Second Time Steps Over 5 Days As demonstrated by Figure 18, accuracy is consistent between timesteps in short term simulations, even a factor of ten difference results in negligible disparities in results. Timesteps up to around 4 minutes (240s) were found to be acceptable. Long term tests were also conducted, comparing the performance of 60 and 240 second timesteps. It was found that over long simulations (1 year), there was significant curve smoothing in the case of the 240s timestep resulting in a 7oC underestimate of peak temperature; this was not apparent in short simulations (Figure 19). Difference in peak temperature between 60 and 30s timesteps was approximately 0.2oC; 60s or less was judged to be an appropriate timestep.

Δt=60s Δt=240s

Figure 19: Comparison of short term (5 day) and long term(1 year) simulations with small and large time steps, 4.2 Comparison of Results to Literature

Estimated Heat Captured by Soil As mentioned in Section 3.0, the resistance network of the UTES fails to capture the thermal memory of the soil; a method was developed wherein heat lost through the surrounding was tracked and then added to the soil temperature monthly. By performing simulations as separate months and then stringing the graphs together Figure 20 was realised. It is clear that as heat is added to the system performance increases – the first month is identical but the blue line begins to pull away after that. However, the performance difference between the two was less than expected, likely due to the thick control volume of soil around the tank which was taken as a 3m thick sleeve.

Table 1 states the values of ground temperature found by:

퐻푒푎푡 푒푛푒푟𝑔푦 푙표푠푡 = 푚푠표𝑖푙퐶푝,푠표𝑖푙Δ푇

(16)

The heat was added for the 3 hot months after June, until the temperature of the tank began to drop in September. If the simulation had run passed November, the soil temperature would be reduced according to the behaviour of the tank.

Table 1: Ground temperature used by month

Time (months) 1 2 3 4 5 6 Ground Temperature 10 15.51 23.58 31.73 31.73 31.73 (oC)

When comparing the figures of Table 1 with values from previous work by (Hooman & Oclon, 2020), the numbers seem to align relatively well. Diagrams depicting the last days of October and November show temperature zones surrounding the tank around 30oC.

Figure 20: Comparison of heat addition method with ordinary resistance network.

Solar Collector and PV Temperatures Figure 21 depicts the temperature distribution of the solar collector and PV panels. The solar collector temperatures are close to what was expected from literature (Li, 2015) and are higher than the PV panels as predicted. However, the PV panel temperatures were much higher than anticipated for two main reasons. Firstly, the tank water feeds directly into the panel cooling system, meaning when the tank is hot, there is no cooling. Second, the modelled design was based off a flat plate collector, meaning it was effective at trapping heat. The PV panels should have some kind of auxiliary water feed, such that they can still be cooled and produce hot water of hot days. Despite this, the heat being captured is not unreasonable, flat plate collectors are known to heat water up to around 100oC (Duffie & Beckman, 2013).

Figure 21:Comparison of the temperature of solar collector and PV panels over model year June 2014 to June 2015

Overall Tank Temperatures Tank temperatures immediately seem higher than one would expect, given that the earlier model of this system, only reached around 45oC (Hooman & Oclon, 2020). However, a number of sources indicate otherwise. The Solar UTES plant in Friedrichschafen had a peak temperature of 85oC, which the author stated was a result of peak summer temperatures (Bodmann, et al., 2005). Similarly the Rostock plant only discharges water from it’s buffer tank to aquifer thermal storage at around 45oC, implying it is stored at a higher temperature in the buffer (Bauer, et al., 2010). By comparison the ~70oC spikes (Figure 22) seem quite reasonable.

Figure 22: One year UTES tank simulation, June to June

5.0 Results The following are the results obtained from simulations, separated by TES type. Overlayed plots are found in

5.1 AGTES

Figure 23: Temperature of above ground tank, simulated over one year

5.2 UTES According to cost estimates from online (Service Seeking, 2020) and over the phone quotes, the price of excavation for UTES with a tank 10m long and 3m in diameter would be in the order of $50,000AUD. The year-round performances are as follows in Figure 24 and Figure 25

Figure 24: One year UTES tank simulation, June to June

Figure 25: One year UTES tank simulation, June to June, with heat addition

5.3 Combined Plots

AGTES UTES

Figure 26: Both Tanks Simulated with no heat addition, with weather data from June 2014-June 2015

AGTES UTES UTES with heat addition

Figure 27: Comparison of UTES with and without heat addition and AGTES. Simulated with weather data taken from Krakow between June 2014 and June 2015

6.0 Discussion Unexpectedly, the AGTES system has outperformed the UTES, even with the heat addition compensation method by a significant margin. In terms of saving cost this is an excellent result; but it may have been the limitations of the model that caused this output due to a number of aspects. These will be discussed within this section.

6.1 Constraints on 1D Steady State model The model chosen for the UTES is relatively effective and gives a good idea of approximate temperatures, however, it fails to take into account the thermal storage potential of the surrounding soil. This issue was attempted to be rectified with heat addition proportional to the energy calculated to pass through the soil in the steady state model. It was somewhat successful, demonstrating around a 5oC boost in peak temperature, but the is no way to know if this is close to the real figure without modelling the non-steady system. The FEA model of this system (Hooman & Oclon, 2020), does seem indicate that a significant amount of heat was being stored in the soil.

6.2 The Role of Forced Convection Despite the AGTES being an excellent candidate for a steady state system, the model did omit one potential issue. It was assumed that the AGTES would be housed in some kind of lean to of shed to protect it from the weather. If this is not the case, then forced convection must be added to the energy balance and in a cold climate like Krakow it is likely to have a significant effect.

6.3 The Weak Effect of the Heat Addition The heat addition in proportion of energy passing through the soil was disappointing in performance, this is due to the still being steady state. Despite adding heat to the soil, this only increased resistance to heat passing through and did not allow the soil to pass heat back into the UTES tank. It is likely a FEA would necessary to effectively simulate this effect.

7.0 Conclusions and Recommendations It is crucial to reduce Greenhouse Gas emissions to reverse the potentially deadly effects of global warming. As more and more governments globally are implementing goal reductions in emissions and are signing to international targets like the Paris agreement, legislation is likely to follow and soon there will be no choice. However, the high price of entry into the renewable energy sector is a major deterrent. Renewable energy systems need to be made, not only cheaper, but more effective, such that they pay themselves back in the short term.

A combined solar PV, solar thermal and underground thermal energy storage system has been proposed (Hooman & Oclon, 2020).Unfortunately, the cost of excavation is high, and for a system like this to succeed, alternative above ground storage may be necessary. This proposed system has been modelled in Python using a range of heat transfer equations to solve a heat balance. The findings are as follows.

Much to the surprise of the author, the AGTES far outperformed the UTES, exceeding peak temperatures by ~10oC and holding heat longer in the colder seasons. The estimated cost difference between the two cases (due to excavation) is $50,000AUD, as such, it may seem clear that AGTES is the recommended technology. However, further investigation is recommended due to two main model limitations:

1. The UTES was modelled under steady state conditions; that is to say, energy in equals energy out. However, a previous model of this system completed with FEA demonstrates that the primary advantage of storing energy underground is the non-steady behaviour of the soil. The effective thermal mass is increased and if energy within the tank is depleted, energy in the soil can assist in recharging it. This issue was attempted to be rectified by adding temperature to the soil proportional to that which was calculated to have passed through it in a steady state model. This process was completed at gaps between smaller simulations, then overall results were strung together, however is only managed a moderate ~5oC increase in temperatures. 2. The AGTES requires significant ground area to be installed and requires at least some kind of modest housing to avoid forced convection. If the AGTES were in an area more exposed to wind, it would have to be remodelled and compared against UTES. Given the $50,000 saving of opting for AGTES, however, it is unlikely that funds for a lean-to or shed would not be able to be raised.

If both of these avenues are thoroughly investigated and the findings do not change significantly, it is almost certain that AGTES will not only be sufficient, but would likely have a better price to performance than UTES

8.0 References Ameri, M., Mahmoudabadi, M. M., & Shahsavar, A. (2012). An Experimental Study on a Photovoltaic/Thermal (PV/T) Air Collector with Direct Coupling of Fans and Panels. Energy Sources, Part A: Recovery, Utilization, and, 929-947.

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Barone, G., Buonomano, A., Forzano, C., & Palombo, A. (2019). Chapter 6 - Solar thermal collectors. In F. Calise, M. D. D’Accadia, M. Santarelli, A. Lanzini, & D. Ferrero, Solar Hydrogen Production (pp. 151-178). Academic Press.

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A.0 Appendices

A.1 Equations

푅푡표푡푎푙,푈푇퐸푆 = 푅푠ℎ푒푙푙 + 푅푔푟표푢푛푑 Where

푟2 푟푔 ln( ) ln( ) 푟1 푟2 푅푠ℎ푒푙푙 = ; 푅푔푟표푢푛푑 = 2휋푘푡퐿 2휋푘푔퐿

푅푡표푡푎푙,퐴퐺푇퐸푆 = 푅푠ℎ푒푙푙 + 푅푔푟표푢푛푑 Where

푟2 ln( ) 푟1 1 푅푠ℎ푒푙푙 = ; 푅푔푟표푢푛푑 = 2휋푘푡퐿 ℎ푎푖푟푆퐴퐴퐺푇퐸푆

A.2 Modification to Solar PV Panel for cooling. The anticipated modification to a regular PV panel and flat plate solar collector to create a cooled solar panel.

A.3 Sample Weather Data

A.4 Python Simulation Code