in this article, Amir illustrates concept of sand-batteries and focuses on domestic-scale product
Normally, the term battery is used to refer to electrochemical cell ; secondary cell or rechargeable such as dry-cell lithium-ion battery used in electric vehicles or wet-cell lead-acid battery used in conventional vehicles , and primary cell or non-rechargeable such as dry-cell alkaline battery or wet-cell lithium thionyl chloride battery
However, the term battery has been used practically instead of accumulator to refer to any device that stores energy ; whether it was electrochemical , mechanical , or thermal in an effort to be recovered efficiently upon necessity
An example of mechanical battery is hydro-electric battery ; multiple reservoirs , different elevations in which surplus electric energy is used to pump water to upper reservoir thus battery is charged as for higher gravitational potential energy as illustrated
In contrast, battery is discharged upon necessity as water is siphoned through turbines to convert gravitational potential back into electricity in an efficient and eco-friendly approach
Thermal Battery
Technically, thermal-battery or thermal-accumulator is any medium with sufficient thermal capacity , hence higher thermal capacities are preferred , other properties are also critical for material selection, for instance ; specific heat of water (4.20 kJ/kg.°C) appears to be sufficient in terms of thermal capacity , but water is not preferred for thermal-accumulator applications beyond sensible heat (>97 °C) upon phase change in which saturated or super-heated steam with lower specific heat requires pressure vessels and creates safety hazard
That’s why, vegetable oils (2.49 kJ/kg.°C) or asphalt (0.92 kJ/kg.°C) are inadequate materials for thermal-accumulator applications ; in which flammabiliy, auto-ignition , and reactivity are critical for material selection
Recently, an emerging european venture-funded start-up have completed engineering and construction of an exceptional thermal-battery system in Kankaanpää in Finland, to be operated by district central-heating provider. This thermal-battery system is composed of cylindrical vessel (volume : 70 cubic meter) packed with 1.0 metric tonne of sand within closed air-loop heat exchange process as illustrated
In reference to physical composition, the term sand refers to solid particles from 0.05 mm to 2.0 mm in diameter, while particles larger than 2.0 mm are called gravel and particles smaller than 0.05 mm are called silt
In reference to chemical composition, sand is composed of large percentage of quartz which is crystalline form of silicon dioxide , thus quartz-rich sands are not pure ; predominantly mixtures of quartz, feldspar, and muscovite
On the other hand, pure silica dioxide is produced industrially and referred to as high-priced "quartz sand" or "silica sand"
Furthermore, specific heat of quartz-rich sand (0.83 kJ/kg.°C) appears to be sufficient in terms of thermal capacity, not to mention high melting point that reaches (1710 °C) which makes it preferred for thermal-accumulator applications as relatively cheap , deployable , and eco-friendly alternative >> non-flammable and nearly inert
In an effort to reverse-engineer this system, high-temperature operability plus resistance to oxidation are taken as basis for selection as for material of construction , thus austenitic stainless steels are ideal for this type of applications
Critical to mention here, max. operational temperature for closed air-loop process is based on electrical resistance duty ; in other words , heating element material of construction , thus metallic nichrome alloys are ideal for (600°C)
Practically, heating elements made of silicon carbide can reach temperatures within (1300 °C) while molybdenum silicide reaches further till (1400°C)
Note that silicon dioxide within quartz-rich sand may convert into traces of elemental silicon as high temperatures enhance reduction and formation of oxides , that's why (600°C) is optimum for this process to reduce battery-fouling and equipment corrosion
Another critical aspect, heat is transferred via specific tube configuration to reduce required insulation, in other words , heat is transferred in excess to sand-bed core while sand-bed margins serve as insulation
According to Chemiprobe, this feature can be achieved via an altered tube-bundle design thus you need to visualize conventional shell-tube heat exchanger , preferably floating-head type , in which tube bundle is made only of boundary tubes within tube-plate layout perimeter
Now visualize this hollow tube-bundle placed inside sand-bed to enclose sand-bed core , it is also recommend to install fins to enhance heat transfer
In reference to standard shell-tube heat exchanger dimensions, thermal battery tube-bundle resembles satndard 42 inch (1.0 m) in diameter for tube-plate layout , as well as 2 inch (5 cm) diameter per tube , besides (2.7 m) for total length ; tubes plus floating head cover
In addition, thermal bricks can be used to further enhance insulation of sand-bed margins , though heat is conserved for several weeks or months
In reference to industrial fan selection, this process is low-pressure closed air-loop ; nearly clean air process ; no particulates , therefore an efficient 18 kW electric motor-driven centrifugal backward-curved fan is adequate to circulate 240 cubic meter of air per minute within fair static pressure for circulation
Nevertheless, fan selection relates to charge time , hence selection of 11 kW electric motor-driven upon low-price priority will compensate for static pressure, charge-time, and discharge-time , not to mention that as air temperature increases , required fan power decreases
In fact, electric heater efficiency is critical for this process , thus convenient ceramic prototype based on cylindrical air-pass divided by separators to achieve max. heat transfer via multiple nichrome or silicon carbide heating elements , as illustrated below
In reference to operations, it is essential to purge heat exchanger tubes via inert nitrogen throughout "thermal storage phase" or the period after full-charge and before discharge , to limit oxidative corrosion. Keep in mind that manual process control is acceptable thanks to simple process and min. safety hazards
Down-Scale
Commercially, business opportunities are limited for industrial-scale thermal batteries upon low utilisation and total capital expenditure , that's why an engineering company have down-scaled thermal batteries into domestic product powered by renewables
Note that main components of domestic product are as illustrated
sand-bed
insulation-bed
water tube-bundle
direct-immersion heating element
Basically, sand-bed have been marketed as non-toxic and recyclable , as well as insulation-bed material which is speculated to be pulverized fiberglass
Notice that water tube-bundle is made of (3 inch) stainless steel U-shaped pipe, while heating element is directly immersed in sand-bed
Critical to mention again, direct-immersion of heating elements in quartz-rich sand within elevated temperatures accelerates oxidate corrosion of elements and formation of elemental silicon hence sand-bend fouling
That's why it is optimum for this process to utilise "bayonet-elements" ; in which heating element is inserted into tubular metallic shell to prevent direct contact with sand-bed thus safely heated to (500°C)
Chemiprobe started "probing" for technical insights about domestic thermal batteries since last month , and continuously networking with professionals in thermal energy systems
Therefore, Chemiprobe is currently on a mission to model an efficient "cost-effective" domestic thermal battery prototype combined with temperature control in which excess heat can be detoured to multiple radiators that serve as heat sink
Another hint for you : think about thermoelectric generators !!
electrochemical cell : cathode, anode, and electrolyte
dry cell : electrochemical cell with solid-phase electrolyte
wet-cell : electrochemical cell with liquid-phase electrolyte
heat capacity : quantity of heat required to change temperature of material , referred to as thermal capacity ; expressed in terms of specific or molar heat capacity
specific heat capacity : heat capacity per mass unit of material (kJ per kg .°C)
water (4.2 kJ/kg.°C) , steam (1.86 kJ/kg.°C) @ atmospheric pressure
molar heat capacity : heat capacity per mole unit of material (kJ per kmole .°C)
sensible heat : heat transfer or release within thermodynamic system in which pressure and volume are approx. constant ; no phase change , in contrast to latent heat
Polar Night Energy Limited is an engineering start-up company , headquartered in Tampere (Finland) ; specializes in thermal-battery systems for central-heating and industrial applications
Vatajankoski Limited is an energy company , headquartered in Kankaanpää (Finland) ; specializes in operations of district central-heating system
Caldera Heat Batteries Limited is an engineering company , headquartered in Fareham (United Kingdom) ; specializes in domestic thermal-battery systems for central-heating applications besides industrial thermal-battery applications