Revised compact thermal storage module

Author: C Dunstan
Research: Google Gemini
Date:March 5, 2026

I said

Assume the temperature of hot water to be delivered is reduced to 45 degrees centigrade. Please recalculate the energy stored and the resulting changes in the quantity of Phase Change Material (PCM) and other parameters including size and energy stored.

Reducing the target temperature to 45°C is a game-changer for the physical size and cost of your system. In the world of heat storage, the narrower the temperature gap ($\Delta T$), the less "work" the system has to do, and the more compact it can become.

By lowering the delivery requirement from 60°C to 45°C, you are effectively reducing the energy density requirement by 33%.

1. Recalculated Energy Storage

We are now calculating the energy needed to heat 250 Litres of water from 15°C to 45°C.

$$Q=m\cdot C_p\cdot \Delta T$$

$$Q=250\text{ kg}\cdot 4.186\text{ kJ/kg°C}\cdot (45-15)$$

• Total Energy Required: $31.4\text{ MJ}$ (or $8.7\text{ kWh}$).

• Comparison: Your previous 60°C target required $13.1\text{ kWh}$.

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2. Resulting Changes in PCM and Volume

Because you are using Sodium Acetate Trihydrate (SAT), which melts at 58°C, your store still needs to be "charged" to at least 60°C to take advantage of the phase change (latent heat). However, because you only need to extract enough energy to hit 45°C, you need much less material.

Parameter	Previous (60°C Delivery)	New (45°C Delivery)	Change
PCM Mass (SAT)	$150\text{ kg}$	$100\text{ kg}$	-33%
PCM Volume	$115\text{ Litres}$	$70\text{ Litres}$	-45 L
Circulating Water	$35\text{ Litres}$	$25\text{ Litres}$	-10 L
Total Internal Volume	$160\text{ Litres}$	$100\text{ Litres}$	-37%

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3. Impact on Tank Size (The "Under-Bench" Shift)

This reduction allows the unit to shrink from a "Tall Tower" to a "Counter-Height" or "Sideboard" unit. If we keep the $600\text{mm}\times 300\text{mm}$ footprint (to stay slim in your hallway), the height changes significantly:

• Internal Height: Drops to approx. $870\text{ mm}$.

• Total External Height: Approx. $1,050\text{ mm}$ (including insulation and the top plumbing manifold).

Pro Tip: At $1.0\text{ meter}$ tall, this unit can now serve as a functional hallway "entry table" or be tucked under a standard kitchen island bench if you have an open-plan apartment.

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4. System Performance Parameters

The lower delivery temperature also makes the Brazed Plate Heat Exchanger (BPHE) more efficient because the "Temperature Difference" ($\Delta T$) between the store ($60^{\circ }\text{C}$) and the shower ($45^{\circ }\text{C}$) is much wider.

• Heat Transfer Power: You now only need to transfer $21\text{ kW}$ to achieve $10\text{ L/min}$ at $45^{\circ }\text{C}$.

• Store Return Temp: With an $800\text{ L/h}$ pump, the water returning to the tank will be approximately $37.5^{\circ }\text{C}$.

• Safety/Legionella: Even though the shower is $45^{\circ }\text{C}$, the system remains safe. Because the BPHE is "instantaneous," no domestic water is stored at that lukewarm temperature where bacteria grow; the water in the tank is a closed loop.

Summary of Benefits for your Apartment:

1. Lower Weight: Total system weight drops from $\approx 225\text{ kg}$ to $\approx 150\text{ kg}$, making it even safer for apartment floor joists.

2. Smaller Footprint: The unit is now significantly less imposing in a small hallway.

3. Faster Charging: Recharging $8.7\text{ kWh}$ of heat via a standard $2.4\text{ kW}$ electric element will take about 3.5 hours (vs. 5.5 hours for the larger unit).

Would you like me to adjust the cabinet design to show how it could look as a $1,050\text{mm}$ "entry console" for your hallway?

I said

Yes. That would be good, thank you.

By shrinking the height to approximately 1,050 mm, the unit transforms from a bulky appliance into a piece of functional hallway furniture. At this height, it matches a standard "tall" console table or a bar-height counter, making it much more palatable for an apartment entryway.

1. The "Entry Console" Layout

To make this work as furniture, the design is split into two zones: the Thermal Core (bottom) and the Service Manifold (top).

Section	Height	Contents
Top Manifold	150 mm	BPHE, Pump, Flow Switch, and Thermostatic Mixing Valve.
Insulation Cap	40 mm	High-density PIR foam board.
PCM Tank	810 mm	100kg SAT modules + circulating water.
Base/Feet	50 mm	Leveling feet and vibration dampening pads.
Total Height	1,050 mm	Perfect for keys, a mail tray, or a lamp.

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2. Strategic Component Placement

In this "Console" configuration, you can utilize the depth and width to keep the unit ultra-slim.

• The "Plumbing Shelf": By placing the BPHE and pump at the top, you ensure that any air bubbles in the system naturally migrate to a single bleed point. It also keeps the "active" parts at waist height for easy maintenance.

• The Facade: You can use a standard kitchen cabinet door (600mm wide) or a custom timber panel. Because the PCM is sealed and the tank is insulated, the exterior of the cabinet remains at room temperature.

• Access: The top "tabletop" surface can be a removable stone or timber slab. If the pump ever needs a look-over, you simply lift the "lid" of the console.

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3. Internal Stacking for $45^{\circ }\text{C}$ Delivery

Since you have a $600\text{mm}$ width, you should stack your PCM modules horizontally like a deck of cards with small spacers (5mm) between them.

1. Water Flow: The $800\text{ L/h}$ pump pushes water into the bottom of the tank.

2. Heat Pickup: The water rises through the thin gaps between the $100\text{ kg}$ of SAT modules, absorbing heat evenly.

3. Extraction: By the time the water reaches the top manifold, it has hit the $60^{\circ }\text{C}$ store temperature, ready to enter the BPHE and flash-heat your shower water to $45^{\circ }\text{C}$.

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4. Technical "Aha!" Moment: The Approach Temperature

By delivering at $45^{\circ }\text{C}$ from a $60^{\circ }\text{C}$ store, you have a $15^{\circ }\text{C}$ "Approach." * In your previous $60^{\circ }\text{C}$ design, the approach was nearly $0^{\circ }\text{C}$, which is physically difficult for a heat exchanger and requires a massive BPHE.

• With a $15^{\circ }\text{C}$ buffer, the Scintex B3-12A 30-plate unit will perform effortlessly. You’ll get "endless" hot water (until the PCM is depleted) with zero temperature fluctuations.

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Summary of the "Console" Build

• Footprint: $600\text{ mm}\times 300\text{ mm}$.

• Weight: $\approx 150\text{ kg}$ (Equivalent to a heavy bookshelf).

• Visual Impact: Looks like a stylish entryway cabinet.

• Utility: Provides $8.7\text{ kWh}$ of energy—enough for two long $10\text{ L/min}$ showers back-to-back with a quick recovery time.

Would you like to discuss the charging side of things? We haven't yet touched on the electric heating element required to melt the PCM and "charge the battery" during off-peak hours.

I said

Yes. Please provide the new cost estimates for the components for the heat storage unit with these operational characteristics.

Reducing the target temperature to 45°C and the capacity to 100 kg of PCM significantly lowers your material costs. However, we now need to account for the "Charging System"—the components that actually melt the PCM and store the energy during off-peak hours.

1. The Charging Side: Adding the Heat

To charge your $8.7\text{ kWh}$ thermal battery, you need an electric immersion element inside the tank.

• The Element: A $2.4\text{ kW}$ (10 Amp) stainless steel immersion heater is standard.

• Charging Time:

  $$t=\frac{8.7\text{ kWh}}{2.4\text{ kW}}\approx 3.6\text{ hours}$$

  This easily fits within any off-peak window (typically 6–8 hours).

• Temperature Control: You will need a Dual-Stage Thermostat.

  1. Stage 1 (Operating): Shuts off the element when the PCM reaches $65^{\circ }\text{C}$.

  2. Stage 2 (Safety): A manual-reset thermal cutout that kills power if the tank hits $85^{\circ }\text{C}$ (to prevent the water from boiling).

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2. Updated Component Shopping List (Console Configuration)

The total cost has dropped because we’ve moved from 150 kg to 100 kg of PCM and a smaller tank.

Item	Specification	Est. Cost (AUD/USD)
PCM (SAT)	100 kg Sodium Acetate Trihydrate (Modules/Bulk)	$300 – $450
Internal Tank	Custom 100L Slimline (Stainless Steel or Heat-Resistant PP)	$350 – $550
BPHE	Scintex B3-12A (30 Plates is now plenty)	$150 – $200
Circulation Pump	Grundfos Comfort or Topsflo TS5 (High Temp)	$150 – $280
Heating Element	$2.4\text{ kW}$ Stainless Steel Immersion Heater (1" or 1.25" BSP)	$60 – $90
Control Unit	Digital Thermostat + 25A Relay + Timer	$70 – $120
Plumbing Kit	Mixing Valve, 2L Expansion Vessel, Flexi-Hoses	$180 – $250
Insulation/Cabinet	40mm PIR Boards + Entry Console Shell	$200 – $350
TOTAL ESTIMATE		$1,460 – $2,290

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3. Key Changes in Strategy

• Tank Construction: With a height of only $1,050\text{ mm}$, the "hydrostatic pressure" (the weight of the water pushing out on the tank walls) is much lower. This makes it easier and cheaper to build the internal tank out of high-temperature plastic (like Polypropylene) rather than expensive Grade 316 Stainless Steel.

• The BPHE Efficiency: Since you only need to hit $45^{\circ }\text{C}$, the heat exchanger will work much faster. You'll get hot water almost instantly, and the pump doesn't have to work as hard to maintain the temperature.

• The "Console" Fit-out: Because the tank is shorter, you have more "airspace" at the top of the cabinet. This allows for better ventilation for the pump and easier access to the electrical terminals of the heating element.

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4. Safety Considerations for Apartments

Since this is an "Active" system (pump and heater) in an apartment:

1. Dry-Burn Protection: Ensure the heating element is mounted near the bottom of the tank so it is always submerged, and include a "low water" float switch if possible.

2. The Relay: Use a Solid State Relay (SSR) for the heating element. It’s silent (no clicking at 2 AM when the off-peak kicks in) and lasts much longer than mechanical contactors.

Summary of Operational Characteristics

• Capacity: ~2 showers back-to-back ($10\text{ L/min}$ for 20 mins total).

• Recharge: ~$1.50$ to $2.50$ (depending on your off-peak rates).

• Maintenance: Check the pump once a year; the PCM should last 10+ years without degradation.

Would you like me to create a "Wiring & Logic" diagram that combines the Charging (Heater) and the Discharging (Pump) circuits into one control box plan?