#3821 Solar-Powered "Heat Motor"
#3821 Solar-Powered "Heat Motor"
Sunlight split in two: Organic layer promises leap in solar power efficiency
https://techxplore.com/news/2025-10-sunlight-layer-solar-power-efficiency.html
#3821 Could the sun cause a loop of coolant to drive a motor?
Yes, absolutely. Using the sun to heat a fluid loop (which could be a coolant) to drive a motor is the fundamental principle behind solar thermal power.
These systems work by concentrating sunlight to create high temperatures, heating a fluid, and then using that heat to produce motion, which can then generate electricity.
Here are the primary ways this is done:
1. Driving a Steam Turbine (Common in Power Plants)
This is the most common large-scale method.
Collect Heat: Large fields of mirrors, called heliostats (in "power towers") or parabolic troughs, focus sunlight onto a receiver.
Heat Fluid: A "coolant" or heat-transfer fluid (often molten salt, synthetic oil, or water) flows through the receiver, absorbing the intense heat and reaching temperatures of 500-1000°F (260-540°C) or more.
Create Steam: This hot fluid is pumped through a heat exchanger, where it boils water to create high-pressure steam.
Drive Motor: The steam is used to spin a turbine, which is a type of motor.
This turbine then drives a generator to create electricity.
2. Driving a Stirling Engine (A True "Heat Motor")
This method often uses a "dish" system and directly drives an engine with the heat.
Collect Heat: A large, dish-shaped mirror concentrates sunlight onto a single point.
Heat Engine: At that focal point is a Stirling engine (sometimes called a "hot air motor").
Drive Motor: The concentrated heat directly heats one end of the engine.
This causes gas inside the sealed engine (like hydrogen or helium) to expand. This expansion and subsequent cooling (at the other end of the engine) drives pistons, which spin a flywheel and a generator.
In this case, the sun's heat directly drives the motor, using an internal working fluid (the gas) rather than a separate coolant loop.
3. Driving a Turbine with Air (Solar Updraft)
This is a less common but fascinating concept that uses air as the fluid.
Collect Heat: A massive, greenhouse-like canopy on the ground heats the air trapped underneath it.
Create Updraft: In the center of this canopy is a very tall chimney.
The hot, buoyant air from under the canopy rushes up this chimney (a "convection current"). Drive Motor: Wind turbines (a type of motor) are placed in the base of the chimney and are spun by this powerful updraft, generating electricity.
In all these cases, the sun's energy creates a temperature difference that causes a fluid (molten salt, gas, or air) to move and do work, driving a motor or turbine.
What would work best as an a "heat motor" add-on to standard solar panels?
Based on the challenge of harvesting heat from standard solar panels, the technology that would work best as a "heat motor" add-on is a Thermoelectric Generator (TEG).
While mechanical heat engines like Stirling engines or Organic Rankine Cycle (ORC) systems exist, they are not a good fit for this specific application. The problem is the quality of the heat. Standard PV panels are designed to stay cool, and their waste heat is very low-grade.
Here is a breakdown of the options and why TEGs are the most practical choice.
The Problem: Low-Grade Heat
A standard PV panel's job is to make electricity, not heat.
Optimal Temperature: 15-35°C (59-95°F)
Typical Waste Heat: On a hot day, the panel's surface might reach 45-80°C (113-176°F).
This is considered low-grade heat, and it's very difficult to convert into motion efficiently.
Candidate 1: Thermoelectric Generators (TEG)
This is the most practical and realistic answer. A TEG is a solid-state device (it has no moving parts) that generates electricity directly from a temperature difference.
How it Works: You would attach the "hot" side of the TEG to the back of the solar panel and a "cold" side (with a heat sink) to the air. The small temperature difference between the panel and the air creates a small amount of voltage.
Pros:
Simple & Scalable: No moving parts, silent, and very low maintenance.
Direct "Add-on": This is the only technology that can be practically applied as a simple add-on to an existing panel.
Dual Benefit: By pulling heat out of the panel, the TEG can slightly cool the PV cells, improving their primary electrical efficiency.
Cons:
Very Low Efficiency: TEGs are notoriously inefficient, typically in the 3-8% range. The power you would get from the TEG would be a tiny fraction of what the panel itself produces.
Best fit for: A realistic, solid-state "add-on" where simplicity and lack of maintenance are more important than high power output.
Candidate 2: Low-Temperature-Difference (LTD) Stirling Engine
This is a true "heat motor," but it's not a practical add-on for this use.
How it Works: A Stirling engine runs on any external heat source.
An LTD Stirling engine is a specific type designed to run on tiny temperature differences—sometimes just the heat from your hand. Pros:
It's a genuine "heat motor" that produces motion from small temperature changes.
Cons:
Extremely Low Power Density: To get any useful amount of power, an LTD Stirling engine needs to be physically enormous. A unit big enough to attach to a solar panel would produce a trivial, unusable amount of power.
Cost and Complexity: It's a complex, finely-tuned mechanical device with moving parts, making it far more expensive and prone to wear than a TEG for the same (or less) power.
Not a Commercial Product: These exist as desktop novelties and in labs, not as commercial "add-ons" for solar panels.
Best fit for: Demonstrations and novelties. It is not a practical power-generating solution for this scenario.
Candidate 3: Organic Rankine Cycle (ORC)
This is a large-scale industrial solution and is not suitable for a single panel.
How it Works: An ORC is like a steam engine but uses a special "organic" fluid with a much lower boiling point than water.
This allows it to run on lower-temperature heat sources like industrial waste heat or geothermal energy. Pros:
It is the most efficient technology for converting low-to-medium grade heat (over 80°C) into electricity on a large scale.
Cons:
Too Cold: The 45-80°C waste heat from a PV panel is still at the very bottom, or even below, the minimum temperature required for an ORC system to run efficiently.
Massive Scale: ORC systems are complex, industrial plants involving pumps, turbines, heat exchangers, and condensers. They are not "add-ons"; they are power plants in their own right, designed for megawatts of heat, not the few watts from a single panel.
Best fit for: Industrial factories, geothermal power plants, or large-scale concentrated solar thermal fields, not standard PV panels.
💡 The Takeaway: PV-T Systems
While a "heat motor" add-on isn't really practical, the concept of using this waste heat is. The real-world application is called a Photovoltaic-Thermal (PV-T) hybrid system.
Instead of trying to turn the heat into more electricity, these systems simply harvest the heat for other uses.
A typical PV-T system runs a liquid coolant (like water or glycol) in tubes behind the PV cells.
Cools the Panel: This makes the PV cells more efficient (up to 10-15% more electricity).
Creates Hot Water: The harvested heat is then used to pre-heat water for a home, swimming pool, or radiant floor heating.
This "cogeneration" approach is far more efficient and economical than trying to convert the low-grade heat back into electricity with a secondary motor.
Would a person-mountable TEG provide energy for small devices?
Yes, but only for very small, low-power devices.
A person-mountable Thermoelectric Generator (TEG) creates power from the temperature difference between your body heat (hot side) and the surrounding air (cold side).
The amount of power generated is currently very small, but it's enough for specific applications.
How Much Power Are We Talking About?
The power output is measured in microwatts (µW) or, in some cases, milliwatts (mW).
1 milliwatt = 1/1,000 of a watt
1 microwatt = 1/1,000,000 of a watt
Research and commercial devices show power densities in the range of 10 to 70 microwatts per square centimeter (µW/cm²) of skin coverage. The power output depends heavily on:
Temperature Difference (ΔT): The bigger the gap between your skin and the air, the more power you get. You'll generate more power on a cold, breezy day than in a hot, still room.
Location: Areas with high blood flow near the skin, like the upper arm or forehead, tend to produce more power than a wrist.
What Devices Could This Realistically Power?
The energy is small but continuous.
Health and Wellness Monitors: This is the biggest application.
Heart rate monitors
Pulse oximeters
EKG/ECG sensors
Continuous glucose monitors
Low-Power Sensors:
Body temperature sensors
Accelerometers (for motion tracking)
Simple Electronics:
LED indicator lights
Some low-power smartwatches or fitness trackers (the Seiko Thermic watch was an early example).
Low-energy Bluetooth beacons.
The Challenge: Voltage
The biggest hurdle isn't just the amount of power (watts), but the low voltage (volts) it produces. The tiny voltage from a TEG is often too low for a standard electronic chip to even turn on.
Therefore, a person-mountable TEG system requires a special DC-to-DC boost converter.
How about charging batteries for night-time use?
Yes, this is a "holy grail" application for wearable TEGs, and the short answer is yes, but only for very specific, ultra-low-power devices.
Here's a breakdown of the math and the reality of the situation.
The Math: Can It Work?
Let's do a simple calculation.
Energy Generation (The "Income"):
A good, practical wearable TEG might generate about 50 microwatts (µW) of power on average over a day. (This is an optimistic but realistic figure).
Let's say you wear it for 16 hours (while you're awake).
Total Energy Harvested: 50 µW x 16 hours = 800 microwatt-hours (µWh).
Energy Consumption (The "Expenses"):
Now, let's look at a very low-power device, like a simple health tracker that only needs to run for 8 hours at night.
Many ultra-low-power sensors and microcontrollers are designed to run in the microwatt range.
Let's say the device's average power consumption is 100 microwatts (µW).
Total Energy Needed: 100 µW x 8 hours = 800 microwatt-hours (µWh).
In this scenario, your energy "income" (800 µWh) perfectly matches your energy "expenses" (800 µWh).
The Reality: What This Means
This calculation shows that it is theoretically possible. You can use the long, slow "trickle" of energy generated during the day to charge a small battery or capacitor, which then has enough stored power to run a low-power device for a few hours at night.
This is the exact principle behind self-powered wearable sensors. The goal isn't to charge a device you'll use for other things; the goal is to make the device's battery self-sufficient so you never have to plug it in.
The Hurdles (Why It's Not Common)
While the math works, it's extremely difficult to achieve in practice, which is why it's still an active area of research:
Ultra-Low-Power Devices Only: We are not talking about smartwatches with bright screens. The only devices this can work for are those specifically designed from the ground up to consume mere microwatts of power (e.g., a simple temperature sensor that "wakes up" for one second every minute).
The "Cold" Side: The TEG only works if there's a temperature difference.
If you're in a hot, humid room (or under a blanket at night), the "cold" side of the TEG warms up, the temperature difference disappears, and your power generation drops to zero. This is a huge engineering challenge. Power Management is Hard: The system needs a highly complex power management circuit that can take the tiny, unstable voltage from the TEG, store it, and "boost" it to a stable, usable voltage for the device and battery, all while consuming almost no power itself.
In summary: Yes, a wearable TEG can charge a battery for night-time use, but only to power an extremely low-power, integrated device (like a basic health monitor) as part of a closed, self-sustaining system. It is not currently practical for charging general-purpose batteries.
Describe feasible and practice applications for desert villages and military personnel
Yes, a wearable TEG (Thermoelectric Generator) has feasible and practical applications for both groups, but it's crucial to understand their limitations. These devices do not generate a lot of power. They are unsuited for high-power tasks like charging a radio or phone.
Instead, their value is in providing continuous, ultra-low power (microwatts to milliwatts) for self-sustaining electronics.
The Critical Factor: The Desert Climate
A TEG's power output depends entirely on the temperature difference (ΔT) between the hot side (your skin, ~37°C / 98.6°F) and the cold side (the air).
Hot Desert Day: If the air is 40°C (104°F), the ΔT is zero or negative. The TEG will produce no power.
Cold Desert Night: If the air is 5°C (41°F), the ΔT is massive. The TEG will produce its maximum power.
Therefore, most practical desert applications rely on harvesting energy during the cool nights to power a device or charge a tiny capacitor for use the next day.
1. 🏜️ Applications for Desert Villages
For a remote village, the focus is on low-cost, maintenance-free, life-sustaining applications. The TEG isn't for convenience; it's for survival and health.
Self-Powered Infant/Elderly Monitor: A TEG-powered device worn at night (when ΔT is high) could charge a small capacitor to power a biometric sensor. This could monitor the heart rate or body temperature of a vulnerable infant or elderly person and send a low-power Bluetooth or LoRa "ping" to a family member's phone or a central receiver if vitals become dangerous.
Personal Safety/Illumination: A TEG could continuously trickle-charge a small battery for a single, high-efficiency LED. This isn't a flashlight, but a personal "be-seen" light for walking at night or a "find-me" light inside a dark dwelling.
Low-Power Water Sensor: A TEG could be integrated into clothing to power a simple, wearable hydration sensor that provides a basic visual or auditory alert (e.g., a "beep") to warn of severe dehydration.
2. 🪖 Applications for Military Personnel
For the military, the goal is reducing the logistical "tail" of batteries. Soldiers carry many pounds of batteries, so any device that can power itself is a huge advantage.
Biometric "Warfighter" Monitoring: This is the most-researched application. A TEG integrated into a soldier's body armor or shirt would continuously power a suite of biometric sensors monitoring heart rate, core body temperature, and hydration levels. This data would be sent in tiny, low-power bursts to a squad leader or medic, providing real-time data on soldier fatigue, heatstroke, or injury.
Critical Backup "Trickle" Charging: A TEG can't charge a radio, but it can trickle-charge a tiny, non-removable backup battery in a critical device (like a GPS or helmet-mounted display). Its job is to ensure that even if the main battery dies, the device never loses its memory, critical settings, or last known location.
Power for "Sleep Mode": Modern electronics are never truly "off." A TEG could provide the "hotel load" (sleep mode power) for a stowed device, keeping it ready for instant use while saving the main battery for active operations.
Silent, Self-Powered Beacons: A TEG could power a "man-down" or location beacon that activates if a soldier is separated or incapacitated. It would have just enough power to send out a periodic, low-power "I am here" signal without needing its own battery.
From <https://gemini.google.com/app/> Google Gemini 2.5 Pro
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