Energy & Power · Study deck

Energy Harvesting: Budgets, Conversion, and Storage

This first route moves from source measurement to energy-neutral ledgers, converter choice, MPPT gates, and storage design.

Battery Bruno is your guide for this deck.

awareharvesting

Battery Bruno, the module guide, in a scene from this chapter.

After studying this chapter

Learning objectives

You will be able to:

Survey ambient energy sources and reject sources that are too weak or intermittent.
Build an energy-neutral ledger from measured harvest and measured load.
Size storage for autonomy, depth-of-discharge, self-discharge, and temperature effects.
Explain why solar, thermal, vibration, and RF harvesting have different design gates.

Commercial supercapacitors range from small cells to packaged modules; they tolerate rapid charge and discharge well, but leakage and low energy density limit how long they can bridge a harvesting gap. Photo: Sechsa, CC0

Major section

Start With a Cloudy Week

A woodland monitor runs well in summer sun and stops during a wet week under leaves.

The field owner needs the device to measure, store, and report through the weakest expected period, not merely to balance energy on an average day.
Include conversion loss and the energy that storage can safely give back.
Harvesting can extend life; it does not create unlimited power or remove upkeep.
The result is bounded by the measured site and storage condition.

Blue disc-shaped supercapacitor with two wire leads, shown beside a smaller green component for scale.

Major section

Minimum Viable Understanding

Harvesting helps only after the load is already low-power.

Source power must be measured at the deployment point, not assumed from a catalog peak rating.
Solar systems are usually sized by winter/dark-period energy, not summer peak output.
MPPT improves harvest only if its own quiescent current and cold-start behavior fit the source.

Major section

Harvesting Architecture

Photovoltaic adds a distinct review condition; cells convert adds a distinct review condition; sunlight to DC adds a distinct review condition.

The ongoing harvesting architecture analysis should preserve all three.
In the visual at the Solar energy harvesting system architecture: a photovoltaic panel feeds an MPPT charge controller that charges storage, with regulation delivering a stable rail to the IoT load diagram, the two labels make the required comparison explicit.
Three labels carry the instructional work in the chart at the Solar energy harvesting system architecture: a photovoltaic panel feeds an MPPT charge controller that charges storage, with regulation delivering a stable rail to the IoT load diagram.

Solar energy harvesting system architecture: a photovoltaic panel feeds an MPPT charge controller that charges storage, with regulation delivering a stable rail to the IoT load.

Major section

Load Policy

Converter detail matters most when the source is weak and the store is nearly empty.

A harvesting power-management IC must cold-start at the measured source voltage, tolerate the source impedance, regulate or gate the load, and expose a useful power-good decision before firmware assumes energy is available.
For small stores, the charge path itself can waste a surprising amount of energy.
Charging a storage capacitor in one large voltage jump loses roughly 1/2 x C x V^2 in the source and switch path.
Step charging splits the rise into smaller voltage increments, so each transfer has a smaller voltage difference and less loss.

A cylindrical supercapacitor with metallic casing and central circular component, alongside a small green electronic component with three pins.

Major section

Source Reality Map

Source type alone does not tell us how much energy reaches storage.

Light is converted across cell area, but current must still travel through that collection grid; the chipped and obscured patches make the effects of damaged or shaded area tangible.
Harvesting technologies are not interchangeable.
Each source has a different failure mode.

A photovoltaic panel converts incident light directly at each cell, but the useful power available to an IoT node still depends on irradiance, angle, shading, temperature, and the converter's operating point. Photo: David Shankbone, CC BY 3.0

Major section

Source Reality Map (continued)

The photograph therefore supports the chapter's next move from a named “solar panel” to a measured source under its real angle, condition, temperature, and converter load.

A hidden assumption between 100s uW and mW could overturn source reality map.
It sets the usable limit for source reality map.
Real delta-T across the module is much lower than surface-to-air temperature difference.
Ambient RF is usually too weak for normal sensor operation.

Major section

Energy-Neutral Ledger

The design is energy-neutral only if the storage state recovers over the chosen accounting window.

The visual next names: Harvested input to show where the review establishes the starting condition; accounting window finally adds a distinct review condition.
They keep energy-neutral ledger tied to observable evidence.
At Supply over the window, the Energy-neutral ledger: harvested energy times conversion efficiency must be at least the load energy plus storage loss over the worst interval, with a reserve to survive the low-harvest window diagram establishes the starting condition.

Energy-neutral ledger: harvested energy times conversion efficiency must be at least the load energy plus storage loss over the worst interval, with a reserve to survive the low-harvest window.

Major section

Phoebe's Field Notes: Why Harvesting Is an Energy Balance, Not a Peak Rating

The mathematical gist: The chapter's sleep, sensing, and radio states total 2.02 mAh/day, or 6.67 mWh/day at 3.3 V.

With two harvest hours and 80% charger efficiency, the source must supply at least 4.19 mW during those hours.
Using the rounded 6.70 mWh ledger, seven dark days need 46.9 mWh delivered and 83.8 mWh of storage after 80% usable depth and 30% reserve.

Energy harvesting source map comparing outdoor solar, indoor light, thermoelectric, vibration, and RF harvesting by power range and availability risk.

Major section

Communication Under Random Energy Dynamics

Source variability changes when the node can transmit, not only how much energy it uses per day.

A conventional battery-powered radio often starts from a fixed transmit-power budget.
The communication design must therefore respect both the channel and the storage state.
Finite storage changes the question.
Remotely powered communication adds one more coordination boundary.

Major section

Worked Example: Outdoor Solar Sensor

The review is not complete until the team measures the actual panel at the site, verifies charger cold start, and tests recovery after a multi-day dark interval.

Sleep: asleep almost the full hour between hourly reports — the 2-second radio burst dominates.
Design decision: The first sizing result is small because the node is already low-power.

Major section

Storage Design

The two views of that disk-shaped form do not show a new storage type.

The metal can does show a different package.
These pictures help you check physical fit, but they cannot tell you leakage, usable voltage range, or whether the store can bridge a long gap.
Storage must absorb the harvester's irregular supply and still serve the load's pulses.
The printed ratings describe individual parts, while the busbars, housings, and terminals reveal what changes when cells become a higher-energy assembly.

A small green supercapacitor with three metal pins next to a blue circular component, illustrating physical forms of energy storage.

Deck summary

Key takeaways

A woodland monitor runs well in summer sun and stops during a wet week under leaves.

Harvesting helps only after the load is already low-power.
Photovoltaic adds a distinct review condition; cells convert adds a distinct review condition; sunlight to DC adds a distinct review condition.
Converter detail matters most when the source is weak and the store is nearly empty.
Source type alone does not tell us how much energy reaches storage.

Retrieval practice

Recall check 1 of 4

Battery Bruno says: answer from memory, then check your reasoning.

Q1What must an energy-neutral ledger compare?

APeak panel power against the best afternoon load case.

BUsable harvest against measured load in the worst interval.

CStored charge against a catalog radio-current figure.

DMean daylight against the battery capacity label.

Show answer

Answer: B The ledger must include conversion and storage limits in the weak period.

Q2A solar-powered node works on a bright afternoon. What condition should determine its storage and workload plan?

AThe best observed harvest without converter losses

BThe presence of a panel as proof of energy neutrality

CThe worst interval of available energy

DThe sensor’s sampling code without storage aging

Show answer

Answer: C The chapter designs around intermittent supply and the workload’s continuing consumption.

Retrieval practice

Recall check 2 of 4

Battery Bruno says: answer from memory, then check your reasoning.

Q3When is an MPPT controller worth testing in a small harvester?

AChoose a panel whenever its catalogue rating is high because that rating predicts system input behaviour.

BWhen source measurements suggest tracking can improve usable harvested energy.

CWhenever the powered load includes a radio, whatever the source response.

DWhen a simple converter already passes all weak-input tests with margin.

Show answer

Answer: B The converter must start and work at the actual source level.

Retrieval practice

Recall check 3 of 4

Battery Bruno says: answer from memory, then check your reasoning.

Q4A harvester has long dark gaps and short radio bursts. What must storage sizing cover?

AThe largest printed storage capacity without a dark-gap or pulse test

BUse only radio pulse current because it represents energy demand through long dark gaps.

CGap autonomy and radio burst current, allowing for leakage and aging

DUse only best daytime harvest because daytime surplus covers storage discharge during darkness.

Show answer

Answer: C Both time without input and pulse support matter.

Retrieval practice

Recall check 4 of 4

Battery Bruno says: answer from memory, then check your reasoning.

Q5Why should a team use different field gates for solar and vibration harvesters?

ABoth are proven by a sunny afternoon.

BBoth are proven by a bench voltage reading.

CBoth deliver steady energy once installed.

DTheir useful input and failure periods differ.

Show answer

Answer: D Each source must be measured under the conditions that limit it.

Q6An indoor solar node is tested only while the lights are on. What evidence is missing?

ABrighter lamp readings while staff occupy the space.

BPanel output during unoccupied hours when lights are off.

COutdoor panel ratings unrelated to the indoor lighting.

DBattery voltage after charging without a dark-period test.

Show answer

Answer: B The weak lighting period may decide whether storage survives.

Print reference

Answers

Answer key.

B · The ledger must include conversion and storage limits in the weak period.
C · The chapter designs around intermittent supply and the workload’s continuing consumption.
B · The converter must start and work at the actual source level.
C · Both time without input and pulse support matter.
D · Each source must be measured under the conditions that limit it.
B · The weak lighting period may decide whether storage survives.