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Final Summary for MBR Space Settlement Challenge: Feasibility Study of Concentrated Solar Power Plant for

a Lunar Application

Sarah Corbet, Joshua A. Keep, Jayden Kovacs, Francisco Macias, Shon Mori, Reece Otto, Augusto F. Moura, Ingo H. J. Jahn

School of Mechanical and Mining Engineering Technical Report 2019/06

School of Mechanical and Mining Engineering, The University of Queensland

St Lucia 4072, Queensland Australia

1 Concentrated Solar Thermal Power for Space Settlements

Executive summary Abundant, reliable, and affordable energy generation and storage is a key enabler for the colonisation of space. One solution to provide this is concentrated solar thermal power, which captures solar energy in the form of heat and then allows this heat to be stored efficiently, for subsequent heating or conversion to mechanical power using a heat engine. In the current project the feasibility of such a concentrated solar power plant for use on the moon was analysed, based on a nominal round the clock power output of 100 kW. To this effect a performance and preliminary design analysis is carried out for all the critical systems in the power plant: The solar field and receiver, the thermal storage system, the heat engine including turbomachinery, and the heat sink. The resulting system, proposed from this preliminary work, consists of a solar field focusing the light onto a falling particle receiver using lunar Regolith as the heat transfer medium. By using in-situ resources for the heat transfer medium and using the same for the thermal storage, minimises the mass of materials that need to be transported to space. For the heat engine a closed loop recuperated Brayton cycle operating with Argon is proposed. By operating between a maximum temperature of 1250 K, limited by the melting temperature of Regolith and sink temperatures as low as 100 K, which can be achieved using radiative coolers, heat engine thermal efficiencies up to 90 % are achievable. These high efficiencies, the ability to extensively use in-situ resources, and comparatively low complexity of the system make concentrated solar power a promising technology. To realise the full potential of concentrated solar thermal requires further work on full system optimisation and detailed design and testing work on the heat engine, falling particle receiver, particle-fluid heat exchanger, and radiative cooler.


Contents Executive summary ....................................................................................................................... 1 1.0 Introduction, aims, and scope ................................................................................................. 3 2.0 Concept development .............................................................................................................. 4

2.1 Background ......................................................................................................................... 4 2.2 Concept overview and division into sub-systems ............................................................... 4 2.3 Key interactions and system performance ......................................................................... 5

3.0 Subsystem concepts ................................................................................................................. 7 3.1 Concentration: Solar Field.................................................................................................. 7 3.2 Concentration: Receiver ..................................................................................................... 9 3.3 Thermal Storage: Use of in-situ Regolith ........................................................................... 9 3.4 Heat Engine: Turbomachinery Sizing .............................................................................. 10 3.5 Heat Sink: Heat rejection approaches .............................................................................. 11

4.0 Technology challenges and opportunities ............................................................................ 12 4.1 Challenges .......................................................................................................................... 12 4.2 Opportunities .................................................................................................................... 13 4.3 Future Work ...................................................................................................................... 13

5.0 Potential impact and opportunities for implementation of results...................................... 14 6.0 Conclusion and next steps ..................................................................................................... 14 Acknowledgments ....................................................................................................................... 15 References ................................................................................................................................... 15


1.0 Introduction, aims, and scope Abundant, reliable, and affordable energy generation and storage has been identified as one of the Space Technology Grand Challenges by NASA (2010). The aim of this project is to assess the feasibility of a concentrated solar thermal (CST) power system as a solution to provide electrical energy and heat to a space settlement. Providing abundant energy will address one of the primary needs of a growing space colony. CST power systems typically consist of a focusing system,receiver, thermal storage, heat engine, and rejection system as schematically shown in Fig. 1. Today, on earth, these systems are reaching maturity with an installed capacity exceeding approximately 5 GW (REN21, 2018). Two of their key advantages is that they can use abundant low cost materials as the thermal storage medium and the charge/discharge process relies on physical processes, making it infinitely reversible.

Figure 1: Schematic of a Concentrated Solar Thermal Power Plant with key sub-systems identified. Also shown is the optional Heating Loop to provide direct thermal heating, for

example for habitats or greenhouses. Considering a space application, for example on a moon, asteroid, or planet, a number of advantages become apparent, compared to alternate power sources or to earth based operation. - in-situ resources, for example locally mined rocks, can be used as the thermal storage medium.

This reduces transport costs. - heat rejection, by radiation at near zero Kelvin. Being able to achieve such low temperatures (e.g.

in the range 100 – 200 K) allows the construction of heat engines with thermal efficiencies greater than 85%.

- for moons and asteroids, the lack of an atmosphere allows more efficient designs. o The lack of an atmosphere eliminates convective heat transfer. This removes one of the

loss mechanisms affecting thermal storage and the design of receivers. o The lack of winds removes wind-loading, allowing for lighter designs. o The lack of wind prevents fouling of mirrors (e.g. by dust), which can lead to solar field

performance deteriorations. - complex systems are restricted to the heat engine alone. This reduces the requirement for an

extensive spares inventory and makes local repair and re-manufacture a viable option. The ability to build a large energy storage using in-situ resources and use thermal energy directly (e.g. to heat habitats or green houses) makes the system especially interesting for settlements on the earth moon, with lunar nights longer than 350 hr. For comparison, to provide 100 kW of power for a

Receiver

Storage System Particle-Fluid

Heat Exchanger

Wout

Heat Engine

Concentration Thermal StorageHeat Engine

Heat Rejection

Heat SinkHeliostats

SunHabitat Heating

Collection


period of 350 hr requires 21 tons of batteries, assuming next generation Li-S batteries with a power density of 0.6 kWh/kg (Bruce, 2012), a factor of two improvement compared to today’s Lithium-ion batteries. In contrast a thermal storage system would require 395 tons of regolith (moon rock), which could be obtained from in-situ resources and approximately 10 tons of equipment for the solar field, heat engine, and heat sink system. Thus, as long as materials for thermal storage can be sourced from in-situ materials, the CST system has a weight advantage and also benefits from lower complexity and increased robustness. The remainder of the document will first discuss the development of the power plant concept, drawing from prior art and analyzing the critical interactions to develop sub-system requirements. Next the operation of the key sub-systems and how these can be optimised for lunar operation is explored. Finally we discuss the challenges and opportunities we have identified for such a system before providing some concluding remarks.

2.0 Concept development 2.1 Background The establishment of a lunar base is the logical precursor to settlement and exploration of the wider solar system. The proximity to earth and resulting shorter distances remove some of the technological challenges and make the moon an ideal proofing ground for habitats, and other technologies that need to be developed for future space exploration (Landis, 1989; NASA, 2004). In addition, once a lunar base is established, using this as a staging post for exploration has notable advantages. For example, considering the fuel required for long distance space travel (e.g. colonisation of Mars), mining this at the lunar surface and then fueling spacecrafts in lunar orbit is substantially more energy efficient due to the lower gravity and escape velocity of the moon (Li et. al. 2017). All these ventures are reliant on the availability of abundant thermal energy to heat spaces, especially during the long lunar night, and abundant electricity to operate services and to allow electrolysis of lunar ice (Mahesh, 2010), to create hydrogen and oxygen for rocket fuel. The sun is one of the most promising energy sources for such an undertaking. However, due to tidal locking between the earth and the moon, the lunar day/night cycle is approximately 27 days, 7 hr, slightly shorter than the time it takes the moon to orbit earth. Thus the critical challenge for any power system is continued power provision during the lunar night lasting up to 350 hr. The two most developed technologies to capture solar energy are photovoltaics (PV) and thermal system. In the PV system, solar energy (photons) are used to knock electrons out of semi-conductors, a process that creates a flow of electrons and thus current. For operation when the sun is not visible, this electrical energy must be stored using batteries. In contrast, a thermal system captures solar energy in the form of heat, which can be stored by heating a storage medium. Then a heat engine is used to convert the thermal energy to mechanical energy, which can be used to drive a generator when needed. The key advantage of the thermal system is the fact that thermal storage is less complex and much cheaper than electrical storage using batteries. 2.2 Concept overview and division into sub-systems A typical CST plant is shown in Fig. 1. To attain maximum system efficiency it is essential to co-optimise all the plant in its entirety. However, for the current preliminary design and feasibility investigation it is more insightful to sub-divide the plant into key self-contained sub-systems and to allow some flexibility at the interfaces. This approach allows parallel analysis of the subsystems and by considering the sensitivity to interface parameters, provides some insight on key interactions and preferential conditions. For our purpose the plant is divided into the following subsystems. (a) Concentration: The heliostat field concentrates the incoming solar radiation and focuses it

uniformly onto the receiver. Through optimizing the field size and layout and developing


appropriate heliostat (mirror) pointing strategies it is possible to maximise the collected energy during a lunar day and to account for different locations (latitude).

(b) Collection: The receiver transfers the focused energy incident from the solar field to a storage medium. Receiver design is a balance between achieving a high temperature for the storage medium and minimizing energy losses.

(c) Thermal Storage: This system acts as the thermal battery of the system. Logically the Concentration and Collection systems will only operate during the day, when the sun is up. During this time some incoming energy will be transferred directly to the Heat Engine, while the remainder of energy is stored in the form of heat. During the night, energy from the thermal storage reservoir is transferred to the Heat Engine, allowing this to continue power production. Typical thermal storage systems consist of two tanks, a hot and a cold tank. Material is moved from the cold to hot tank via the receiver to collect energy, and fluid is moved from the hot to cold tank to supply the Heat Engine.

(d) Heat Engine: This is the heart of the power plant, converting thermal energy to useful mechanical work (e.g. a rotating shaft), which is used to do work or coupled to electricity generation systems. Heat engines usually employ Rankine or Brayton cycles, as used in thermal power stations on earth, which can be optimised for the application. For most efficient energy conversion it is desirable to maximise the Thermal Storage temperature and to minimise the Heat Sink temperatures.

(e) Heat Sink: To ensure operation of Heat Engines it is essential to provide a place to reject heat. As aforementioned it is desirable to do this at the lowest possible temperature.

(f) Habitat Heating (optional): Having a Thermal Storage reservoir at high temperature also creates the ability to provide direct heating, for example for habitats or greenhouses. Depending on required heating duty, the upstream subsystems (Concentration, Collection, and Thermal Storage) would have to be appropriately oversized.

Functionally these subsystems can be described: the heliostats concentrate the incoming solar radiation to create a concentrated energy flux; the receiver uses this incoming energy flux to heat the storage medium to a high temperature; the storage system allows the thermal energy to be stored until required; the heat engine converts the thermal energy to useful mechanical work; the heat sink rejects any energy extracted from the storage and not converted to mechanical work output. 2.3 Key interactions and system performance To understand the overall system it is beneficial to look at the overall system and to analyse how losses accumulate from the incoming radiation, IR, to the useful mechanical work, M, that can be extracted from the heat engine. A cascade showing the energy flow and the effect of losses is shown in Fig. 2. Considering this, it can be deduced that (a) it is desirable to optimise all efficiencies, and (b) the efficiency of the heat engine, hengine, has the highest impact when trying to minimise system size and cost. In order to compensate for a poorly performing heat engine, all the upstream systems (heliostat field, receiver, storage size) need to be increased in size, so that the energy transferred to the heat engine, E, can increase. In contrast, a reduction in field efficiency, hfield, only requires the field size increase in order to maintain the same energy being transferred to storage, ES.

Figure 2: Cascade of energy and accumulation of losses (not to scale).

Incoming Radiation flux,

IR

Energy to Storage, ES

ES = hfield hreceiver IR Areafield

Energy to Heat Engine, E

E = hfield hreceiver hstorage IR Areafield

Field and Receiver Losses Storage

Losses To Heat Sink

Useful Mechanical Work, M M = hfield hreceiver hstorage hengine IR Areafield


Looking at the above energy cascade alone, and the fact that heat engine efficiency, hengine, is limited by the Carnot efficiency given as 𝜂"#$%& = 1 − +,

+-, it would suggest that maximising the storage and

receiver temperature would lead to the optimum configuration, especially as the increase in storage temperature difference allows the mass or required storage medium to be reduced. However, as shown by Binotti et. al. (2017) for earth based systems there is a practical upper temperature limit of around 1000 K, above which convective and radiative losses from the receiver and thermal storage start to dominate. In space based applications with a rarified atmosphere the convective losses are eliminated and higher temperatures can be targeted.

(a) Schematic of heat engine concept

(b) Temperature entropy diagram

Figure 3: Schematic of Recuperated Brayton Cycle (RCBC) and example results for RCBC-1250-200 cycle corresponding to operation with source and sink temperatures or 1250 and 200 K respectively.

To understand the impact of temperature selection on heat engine performance and thus on the remainder of the system, a systematic cycle analysis for the heat engine component was conducted using the software SSCAR (Jahn, 2017). For this purpose a simple Recuperated Brayton Cycle (RCBC) as shown in Fig. 3(a), using Argon as the working fluid was considered. This cycle configuration was selected due to its comparatively low complexity and high efficiency and Argon was used due to its inertness and low critical point, essential for operation at low temperatures. For this analysis the source temperature range was defined as 1000 - 1500 K, corresponding temperatures slightly above the optimum for earth based system and the a material limit based on aerospace grade gas turbine superalloys. For the sink a conservative temperature of 300 K, substantially above the lunar sub-surface temperature, and an ambitious temperature of 100 K were selected. For this analysis the heat exchanger approach temperatures were defined as 5 K, compressor and turbine efficiencies set to 80 % and 90 % and the remaining parameters (pressure ratio and mass flow rate) adjusted in order to achieve an output power of 100kW and peak efficiency. Results, in the form of a temperature-entropy diagram for the mid-point cycle RCBC-1250-200 with a source temperature of 1250 K and a sink temperature of 200 K, are shown in Fig. 3(b). The performance of the remaining cycles is summarised in Fig. 4 and Tab. 1. Figure 4(a) clearly shows that cycle efficiency is predominantly a function of sink temperature and that source temperature, while still being beneficial has a lesser effect. Furthermore, the data in Fig. 4 (b) & (c) highlights the significant benefits of a heat rejection system that can achieve low temperatures (200 K or less). By achieving low temperatures the required input power is reduced by a factor of two and the amount of heat rejected by almost a factor of 20.


(a) Cycle Efficiency

(b) Power extracted from

thermal storage (c) Power rejected to heat sink

Figure 4: Critical heat engine performance parameters as a function of source temperature (Particle-Fluid Heat Exchanger supply temperature) and sink temperature (Heat Sink return

temperature). Table 1: Cycle parameters for providing an output power of 100kW.

Cycle (Type-SourceTemp-SinkTemp)

Argon mass flow rate (kg/s)

hengine (%)

hideal (%)

Shaft Power (kW)

Power from Storage (kW)

Required storage mass. Regolith (tons)

Heat Sink Power Rejected (kW)

Inlet Temp (K)

Outlet Temp (K)

RCBC-1000-100 0.86 86.0 90.0 100.0 116.2 - 16.2 138.4 105.0 RCBC-1000-300 1.65 44.7 70.0 100.0 223.4 - 123.4 448.2 305.0 RCBC-1250-200

0.81 72.4 84.0 100.0 138.2 395 38.2 294.9 205.0

RCBC-1500-100 0.53 90.9 93.3 100.0 110.0 - 10.0 138.1 105.0 RCBC-1500-300 0.76 63.9 80.0 100.0 156.4 - 56.4 447.9 305.0

Table 1 summarises the critical parameters for selected cycle configurations and shows that with low sink temperatures the heat engine efficiency, hengine, approaches the ideal Carnot efficiency, hideal, and that using low sink temperatures has significant benefits on the mass of the thermal storage and the power being rejected by the heat sink. For further analysis of subsystems we focus on the intermediary, RCBC-1250-200 configuration, corresponding to a source and sink temperature of 1250 K and 200 K respectively. Depending om performance trends of other systems, a move towards RCBC-1500-100 configuration would be desirable.

3.0 Subsystem concepts 3.1 Concentration: Solar Field The purpose of the solar field is to concentrate the incoming light, measured as Direct Normal Irradiation (DNI) and to concentrate it so that high fluxes and thus high temperatures can be reached at the receiver. An extensive review of these technologies is provided by Lovegrove & Stein (2012). The most common techniques in order of increasing concentration ratio are Parabolic Trough, Linear Fressnel, Central Tower Receiver, and Parabolic Dish. For a space application, where the rarified atmosphere eliminates convective losses and where low weight and low complexity are desirable, the Central Tower Receiver, as shown in Fig. 5 (e), is a preferred solution. Especially as it allows for falling particle receivers, which can use Regolith particles as the heat transfer medium, maximising use of in-situ materials.


(a) Solar field efficiency at 105.6 hr after dawn

(b) Solar field efficiency at mid-day, 177.6 hr

after dawn (peak efficiency)

(c) Solar field efficiency at 283.2 hr after dawn

(d) Flux distribution on receiver at mid-day

(peak efficiency)

(e) Central tower receiver concept

(f) Variation of total heat flux at receiver over one lunar day

Figure 5: Performance of the optimised solar field. (a) to (d) show snapshots of solar field and receiver heat flux for solar plant located at the equator. (f) shows the effect of latitude on total heat

flux received. Designing a solar field is a multi-variable, where the size and layout of the solar field needs to be adjusted, so that a uniform heat flux is obtained at the receiver. Challenges arise from the variation in sun ray directions throughout the lunar day, heliostats being in the shadow of one another, and imperfections in the heliostat mirrors that limit focusing ability over long distances. For the preliminary field design we have used software SolarPilot (NREL, 2018), which allows the optimisation of solar fields for terrestrial applications and this process is described in more detail by Corbet & Jahn (2019). For the feasibility study on the moon, sun data was taken from simulations by Kaczmarzyk et. al. (2018) and several non-relevant loss-models in SolarPilot were disabled. The results from a solar field, optimised to maximise cumulative energy collection, while maintaining a uniform illumination of the receiver and limited peak power to less 3000 kW is shown in Fig. 5. The results show that through appropriate sun tracking of the individual mirrors (see Fig. 5 (a)-(c)) a

Sun Rays Sun RaysReceiver

HeliostatTower


uniform heat flux on the receiver surface, is maintained (see Fig. 5 (d)) and the power output is manimised by ensuring a flat power profile around mid-day as shown in Fig. 5 (f). Figure 5 (f) also shows the variation in receiver absorption as a function of latitude. For this analysis, a new field was optimised at each location with a goal of limiting maximum absorption rate to 3000 kW. From the data it is clear that the maximum absorption rate can be maintained at latitudes up to 80°, but that the cumulative performance over the day deteriorates further away from the equator. Considering the mass of materials required to construct the field, this increases from 0.053 to 0.108 kg/kWh as latitude is increased. While the above delivers a collection system with peak output of 3000 kW, a study was also performed for a 300 kW peak output field to match with the 100 kW heat engine. Performance is qualitatively identical to what is shown in Fig. 5 (f), but the specific weight increases to 0.097 kg/kWh for an equator location, showing that there is a significant advantage in constructing larger plants. 3.2 Concentration: Receiver The objective of the receiver is to convert the concentrated light flux and to heat a thermal transport medium. In a space based application this process is simplified due to the rarified atmosphere which removes convection, typically the dominant loss factor at high temperatures. Most commonly receivers use water/steam, molten salt, or sodium as the heat transfer medium, but more recently the development of falling particle receivers has received increased attention, and operation at temperatures in excess of 1000 K have been demonstrated (Ho et. al., 2016). Using falling particle technologies is especially relevant for space based applications, as wind, which leads to loss of particles, does not exist, the particles can be sourced from in-situ Regolith and using the same medium for receiver and thermal storage minimises losses. As the operating requirements for the receiver are within the ranges of what is covered in terrestrial research no further in-depth analysis was conducted. The next stage would be to complete a detailed design, taking advantage of the absence of wind and the lower gravity. 3.3 Thermal Storage: Use of in-situ Regolith There are three main thermal energy storage options for concentrated solar power: sensible, latent and chemical storage. The most suitable option, especially when operating in conjunction with a falling particle receiver is sensible energy storage and to use in-situ Regolith as the storage medium. Chemical or latent energy storage would require materials to be transported or locally produced, thereby increasing complexity and cost. To understand the implications of using lunar Regolith as a storage, a sizing calculation was performed by Otto & Mori (2018), based on the RCBC-1250-200 cycle from Tab. 1. To produce 100 kW for the duration of the lunar night (354.4 hr), this cycle requires 4.89 MWh of stored thermal energy. Allowing for temperature separation of 50 K at the entry and exit of the particle-fluid heat exchanger and assuming a mean specific heat capacity of 1.351 kJ/kg.K for Regolith, based on the correlation from Colozza (1991), this results in a particle mass flow rate of 0.31 kg/s. Over a lunar night this equates to total of 395 tons, which is equivalent to a volume of 238 m3, using density values provided by McKay & Ming (1990). Using cylindrical tanks, with an equal height and diameter, resulting in the optimum volume to surface ratio so that heat losses are minimised, this yields a tank with diameter and height of 6.72 m. The final storage system would require two of these tanks, for the hot and cold Regolith particles respectively. One way to construct these tanks would be to repurpose empty rocket stages or fuel tanks. Alternatively there are specifically constructed underground storage tanks with appropriate insulation or use existing caves if available. A way to reduce the quantity of Regolith required would be to increase the temperature in the hot storage tank. However, this is limited by the melting temperature of Regolith at 1373 K as reported by Colozza (1991).


3.4 Heat Engine: Turbomachinery Sizing Considering the heat engine the Recuperated Brayton Cycle already offers efficiencies close to the ideal case, especially when using heat sink temperatures of 100 K. For the high sink temperature configurations (300 K), some efficiency improvements can be expected through using more complex cycle configurations. The objective of the heat engine is to convert the thermal energy, available from the thermal storage system, into useable mechanical power. Typically mechanical power is extracted from the heat engine in the form of a rotating shaft, which can be used to drive an electrical generator or be used as a direct input to other mechanical systems. The heart of the heat engine are the compressor and turbine, which compress the cold fluid and the turbine(s), which extract shaft work by expanding the hot and high pressure fluid. Preliminary sizing of the compressor and turbine, for the range of anticipated inlet temperatures reported in Tab. 1, were performed to assess the technical challenges associated with constructing the proposed 100 kW heat engine. The details of this sizing study are reported by Keep (2019). For the purpose of preliminary sizing, approach of specific speed and specific diameter scaling is applied (Whitfield & Baines, 1990), which allows optimum compressor and turbine geometries and efficiencies to be estimated using non-dimensional performance maps, for example as reported by Balje (1981).

(a) Single shaft

(b) Twin shaft Figure 6: Different layouts (a) using single shaft for compressor and turbine, and (b) using a tow

turbines and a split shaft. Option (b) allows the Power turbine to operate at generator speed. For the turbomachinery sizing, tow turbine layouts, as shown in Fig. 6 were analysed. For both layouts and combinations of inlet conditions, radial compressors and turbines were identified as a preferred architecture, as they offer high efficiencies and have secondary benefits, such as being more compact and being more robust to off-design operation. For the reference cycle RCBC-1250-200, the single shaft design resulted in a compressor and turbine with diameters of 61 and 150mm, both operating at 63000 RPM. The twin shaft layout with a compressor and turbine of diameters of 74mm operating at 52000 RPM and a 156mm power turbine operating at 56000 RPM. Considering these layouts, the twin shaft configuration has the potential to yield a slightly more efficient power turbine, which does the majority of the work extraction. However, for the 100kW size currently investigated and the importance of reliability (i.e. reduced complexity) there is merit in favouring the single shaft design. Considering the component sizes and operating speeds, these are within the engineering experience of Auxiliary Power Units (APU) used in aircrafts. However, special consideration would have to be given to the seals and gas foil bearings, which are required for the reliable operation with Argon across the wide temperature range specified by the heat engine. A more extensive discussion of these aspects is provided by Keep (2019).


3.5 Heat Sink: Heat rejection approaches As shown in Fig.3, the effective removal of heat and the ability to cool the working fluid to a low temperature is critical for the efficient operation of the plant. Depending on the operating temperature of the radiator, a range of heat rejection mechanisms are plausible. As shown in Fig. 7, for temperatures above approximately 300 K, s suitable use of the heat is to heat habitats or greenhouses. For temperatures between the mean subsurface lunar temperature (250-260 K for the moon Langseth et. al. 1976) and 300 K, heat rejection using underground pipes is viable. However, for the lowest temperatures radiative coolers are required, which remove heat through radiation to space.

(a) Heating of habitats or

Greenhouses (b) Underground heat rejection using buried pipes

(c) Radiative heat rejection using

Figure 7: Different concepts of heat sink heat rejection systems. Considering the heat sink inlet temperature data presented in Tab. 1 it is evident that only heat rejected from low efficiency design has the potential to provide heating as shown in Fig. 7 (a). However, for these types of applications it is more efficient to use thermal energy directly, as indicated by the heating loop shown in Fig. 1. For the more efficient operating points, the radiator inlet temperature reduces to 294.9 K and approximately 138 K. Here cooling can be achieved through a cascaded under-ground and radiative or radiative only system. The limitation of the underground heat rejection system arises due to the regolith being a good insulator and ground layer affected by day-night temperature variations being limited to a depth of less than 0.05 m (Langseth et. al. 1974). Below this depth, the quasi-constant ground temperature is around 250 K. With these limitations, a horizontal tube style underground heat sink arrangement is preferred, also the need for deep wells is avoided. Preliminary designs for such a system are reported in Macias et. al. (2019) and summarized in Tab. 2. To achieve the lowest temperature and thus the highest cycle efficiencies requires the use of radiative coolers that can reject heat through radiation to space. Such radiators would require shielding from the sun during day-time operation, which can be achieved through locating the radiators inside craters close to the lunar poles that are permanently shaded [Ref], or by the construction of specific sun shields. Alternatively, the heat engine could operate in a dual mode,

Ground

Habitat or GreeHouse

GroundDepth, h

Ground

Incoming Solar Radiation

Natural or Artificial Shade Wall

High Emissivity CoatingLow Emissivity CoatingWorking Fluid


where heat rejection could be achieved through underground systems during the day and switch to radiative during the night where cycle efficiency is critical. A concept radiative cooler is shown in Fig. 7 (c), which is further described in Macias et. al. (2019). Table 2 summarises the operation of the different systems and gives a first indication of the required dimensions of the underground pipe for conductive heat rejection and the radiative cooler. Here the impact of the low conductivity of the Regolith is clearly evident, resulting in prohibitively long underground pipes. In contrast the radiative cooler shows much potential, making this a prime target for further analysis and optimization. Table 2: Comparison of different cooling systems to provide a heat sink for the heat engine. (*based on non-optimised radiative cooler.) Cycle: Configuration Duty Comments RCBC-1500-300 Heating of habitats

or greenhouses 56.4 kW for heating

Re-use conventional finned tubes or similar architectures.

RCBC-1250-200 Cascaded underground and radiative cooler

16.9 kW to underground 21.3 kW radiative

approx. 42 km of 0.1 m diameter underground pipe and 470 m of 0.1 m radiative pipe*

RCBC-1250-200 Radiative cooler 38.2 kW radiative 650 m of 0.1 m radiative pipe* RCBC-1500-100 Radiative cooler 10.0 kW radiative 2800 m of 0.1 m radiative pipe*

4.0 Technology challenges and opportunities The preliminary work completed so far has identified a number of technical challenges that need to be overcome before this concept can be realized. In addition, we have identifed a number of nonintuitive opportunities to further enhance the performance of this CST power system. We will provide a short overview of these challenges and opportunities, followed by a list of critical future work to de-risk such a project. 4.1 Challenges The following challenges, limiting the performance of the proposed system were encountered: Low thermal conductivity of lunar Regolith: Due to the low conductivity only a very thin surface layer (less than 5 cm) of Regolith is heated and cooled during the day night cycle (Langseth et. al. 1974). This is an insufficient layer thickness to act as a transient heat source or heat sink. For the same reason the performance of the underground heat sink is exceptionally poor, meaning multiple 10s of kilometers ??? of large diameter underground pipes are required to reject heat. The material requirements, including the amount of working fluid required to fill pipes makes this unfeasible. Dimensions of heat sink: The preliminary analysis has shown that heat rejection is likely to be one of the challenges for this type of power plant. The reliance on conductive heat transfer to sub-surface rock or radiation yields heat exchangers with prohibitive dimensions. Shading for radiative cooler: The operation of the radiative cooler during day-time requires appropriate shielding from the sun. While craters that are permanently shaded exist at the lunar poles, these locations also suffer from low solar field performance. The development of a locally deployable shading system is highly desirable. Particle fluid heat exchanger: There is no proven technology for a particle-fluid heat exchanger that operates in a rarified atmosphere and low gravity environment. For the current study a conservative approach temperature of 50 K was assumed. Improving this would allow for higher heat engine source temperatures (better cycle efficiency) or lower storage temperatures (lower thermal losses).


4.2 Opportunities The following additional opportunities to further enhance the system were identified: Finned tubes for sub-surface heat rejection: The ability to reject heat underground is limited by the low conductivity of lunar Regolith. A possible way to address this would be to add fins or similar features to the pipe surface that increase the reach and effective surface areas of the pipes. Sub-surface storage tanks for hot/cold thermal storage medium: The low conductivity of lunar Regolith makes it a suitable material for insulation. This raises the question whether Regolith could be used for the construction of thermally insulated storage tanks. These could be on the surface, or sub-surface (e.g. using existing caves). Co-optimisation of heat engine and heat sink for size minimization: So far heat engine and heat sink were analysed independently, to gain an understanding of how each independently vary with different discrete temperatures. As heat sink size (mass) is one of the dominant costs for the system, a promising direction for further research is to co-optimise heat engine and heat sink size (or mass). 3D printing key structures using lunar Regolith: The European Space Agency and DLR have shown that with its high metallic oxide content lunar Regolith can be 3D printed using a sintering process (Meurisse, 2018). This creates the opportunity toincrease the use of in-situ resources for the construction process, for example, receiver tower, heliostat mounts, thermal storage tanks. Explore opportunities of a dual mode or hybrid plant: The key advantage of the thermal power plant is to utilise the in-situ Regolith and then to use this for provision of thermal and electrical energy at night. An alternative to providing an effective radiative heat sink during the day is to either design a dual mode heat engine, that would operate with a lower efficiency, but higher sink temperature during the day or to combine the thermal plant with a photovoltaic plant. Here the photovoltaic plant would provide energy during the day and the thermal plant would operate at night. 4.3 Future Work The following activities would allow notable performance improvements and de-risking of the real project; 1) Optimization, design, and testing of heat engine. The Argon heat engine presented in this work (see section 2.3 and 3.4) uses a relatively simple architecture and so far cycle design and optimisation have been restricted to the heat engine in isolation. It is expected that more complex cycle arrangements (e.g. recompression cycles or partial cooling cycles or different working fluids) will yield higher efficiencies. Similarly, co-optimising the cycle together with the heat sink system will result in a more compact and lighter overall system. Once an optimal cycle is identified, technical challenges remain, particularly with respect to the turbine, compressor, bearings, and seals. Thus an important future program is to design, build, and test a prototype heat engine. This will mitigate technical risks, improve our understanding of heat engines operating at cryogenic conditions, and may identify further opportunities to improve performance. 2) Particle-fluid heat exchangers for low gravity and rarified atmospheres. Effective energy transfer from the particles in the thermal storage to the heat engine working fluid in the heat engine is essential. Current solutions such as fluidized beds are not viable in places with rarified atmospheres. Further development of this critical process is essential. 3) Design and optimization of a high capacity radiator for surface based heat rejection

including integrated radiation shields A preliminary concept for an efficient radiator is presented in section 3.5. These can be positioned either in a naturally shaded location or require an integrated radiation shield. It is expected that this


concept, especially when integrated with a radiation shield, can be significantly improved through shape and topology optimization. Further development of this radiator concept would develop a useful technology with applicability to a wide range of space applications that require cooling to cryogenic temperatures. 4) Design and optimization of a falling particle receiver for low gravity and rarified

atmospheres The low gravity and thus comparably reduced falling speeds for particles, as well as the lack of an atmosphere which removes aerodynamic drag and heat loss by convection, affect how falling particle receivers work. Thus re-visiting the receiver design, with these changes in conditions in mind, would likely boost performance. This should also include appropriate subsystem testing. 5) Light-weighting of hardware and increased use of in-situ resources Currently weight estimates provided for components are based on conventional terrestrial designs. Changes in design requirements, possible through reduced gravity and different environmental conditions (e.g. absence of windloads) and use of advanced materials and concepts (e.g. composites, nano-materials, stretched membranes to replace mirrors, support structures 3D printed from lunar Regolith) will allow significant reductions in component weights.

5.0 Potential impact and opportunities for implementation of results This feasibility study showed that using Concentrates Solar Thermal is a promising technology for provision of space based power. The key advantage is that lunar Regolith, available in-situ, can be used for thermal storage medium. Especially for locations such as the earth moon, where power must be provided for nights that are over 350 hr long, finding a technical solution that can be constructed from in-situ resources is imperative. Using lunar Regolith as the thermal storage medium has a number of advantages including its high melting point and the ability to 3D print structures in Regolith using sintering (Meurisse, 2018), which means storage tanks can be made from solid Regolith, thereby eliminating possible compatibility issues (e.g. corrosion) and allowing easy in-situ repairs. A further advantage of the thermal system, which has not been explored in detail is to use the thermal storage medium to provide heating. For example, in order to maintain high heat engine efficiencies, the temperature in the cold storage tank will be above 900 K. This is hot enough to heat habitats or green houses during long lunar nights. Hence, by appropriately scaling up the solar field and receiver, heating for a lunar base could be provided without the need to increase the thermal storage system. As this heating process (sun à thermal storage à heating) is more direct, than an electrical process (sun à electricity à heating), much higher efficiencies can be achieved. And finally there is the option to hybridise the system with a photovoltaic power plant, so that photovoltaics are used to provide electricity during the day and so that the concentrated thermal plant can provide electricity at night and heat around the clock. Much of the proposed work also has relevance for earth based applications. For example, to provide power in remote locations that also experience extreme changes in ambient conditions between day and night (e.g. deserts). With this in mind there is the potential to build a small (100 kW) proof of concept plant that uses the proposed technologies, including sintering of sand (Kayser, 2018) to construct as much of the plant as possible. This would improve our understanding of the system and de-risk the technologies before trying to build a prototype plant on the moon. At the same time, an environmentally friendly power solution for remote locations would be developed.

6.0 Conclusion and next steps A feasibility study for a Concentrated Solar Thermal (CST) power plant to be located on the moon was conducted. This study focused on the critical components of the plant, to understand their individual performance requirements, and to identify system interactions that give guidance towards an optimal overall system. The aim for the system was to provide 100 kW of mechanical power, for


the duration of a 354 hr long lunar night. The key systems considered were the solar field and receiver, thermal storage, heat engine including preliminary sizing of turbomachinery components, and heat sink. Overall it can be concluded that such a CST system has much potential, especially as lunar Regolith, sourced from in-situ resources, can be used in conjunction with a falling particle receiver and as the thermal storage medium and for the construction of infrastructure, using sintering and 3D printing approaches. This use of in-situ resources, reduces the amount of equipment that must be brought to the space settlement, and allows for easy repair. Furthermore, using thermal storage is technologically less complex than chemical or electrical storage, providing increased robustness. With respect to the subsystems it is shown that creating an efficient heat engine is critical to delivering high overall system efficiency and compactness. To achieve this requires a high temperature heat source and low temperature sink. For the heat source, using a heliostat + central tower arrangement, together with a falling particle receiver, and Regolith particle storage is the preferred option due to the high temperature capabilities and ability to use in-situ resources. For the heat sink, underground heat sinks and radiative sinks were analysed. The performance of the underground sink is comparatively poor due to the low conductivity of lunar Regolith, thus requiring 10s of kilometers of buried pipes. The radiative heat sinks showed promising performance and an ability to reach temperatures down to 100 K, but require shading if operating during the day. When considering the heat engine, the preliminary analysis has shown that a relatively simple Recuperated Closed Loop Brayton Cycle operating with Argon is a promising solution that can achieve thermodynamic efficiencies up to 90%. Considering the concept arrangement with a work output of 100 kW this results in compressors and turbines operating at 64000 RPM, a speed range that falls within the realm of auxiliary power units currently used by the aviation industry. However the wide temperature ranges of the cycle (100 K to 1250 K) may pose poses significant challenges for components including seals and bearings. Considering future work a detailed list of proposed activities is provided in section 4.3. Of these the co-optimisation of the heat engine and other systems, especially the heat sink, is essential to allow the optimum concept and operating conditions to be defined more precisely. Once this is complete, detailed design and testing of the heat engine, particle receiver, and particle-fluid heat exchangers would allow the project to progress further.

Acknowledgments This project received seed funding from the Dubai Future Foundation through Guaana.com open research platform.

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