11 Energy Harvesting: Budgets, Conversion, and Storage

11.1 Overview

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

This is part 1 of 2. Continue with Energy Harvesting: Source Reality and Field Proof for the second focused route.

11.2 Start With a Cloudy Week

11.2.1 Budget the Dark Days First

A woodland monitor runs well in summer sun and stops during a wet week under leaves. The panel’s best afternoon output hid the real design question. 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.

Make a daily energy ledger. Measure what the source supplies at the installed angle and shade. Include conversion loss and the energy that storage can safely give back. List sleep, sensing, local work, sending, retries, and upkeep as loads. Keep units and time periods consistent. Reserve enough storage for the named run of dark or quiet days.

Then replay the weak period. Cover the source. Lower the temperature. Age the storage assumption. Force extra retries. Start from a partly charged state. Check which work is reduced first, whether critical records survive, and whether recovery avoids a burst that drains the store again. Record the lowest state and the lost services.

Harvesting can extend life; it does not create unlimited power or remove upkeep. The result is bounded by the measured site and storage condition. The deeper sections compare sources, converters, storage, duty choices, energy-neutral balance, and the field evidence needed before claiming long-term operation.

Ask the ledger simple questions. What enters on the worst day? What is lost in conversion? What can the store safely hold? What does sleep cost? What does one reading cost? What does one send cost? How often will retries occur? Which task can wait? How many dark days must the device survive?

Measure each answer in the same energy unit and time span. Keep the source trace from the real site. Keep load traces from the real code and board. Use the weak week, not the best hour. State the allowed lost work. Set a low-energy mode and test entry and exit. Repeat as storage ages or the site changes.

Energy harvesting looks easy on a sunny bench and hard during a cloudy week, a dim corridor, or a vibration pattern that stops overnight. The design question is whether the harvested trickle, storage buffer, and load policy can survive the weak periods.

Start with measured source availability, then size storage and duty cycle for energy-neutral operation instead of peak conditions.

Battery Bruno

“Every milliamp is a day of battery — budget the sleep before you budget the features.”

In this chapter, Bruno audits the harvest side too: what the trickle supplies, what the load draws, and how much storage buys the dark days.

In 60 Seconds

Energy harvesting is viable only when measured harvested energy, after conversion losses and storage limits, exceeds measured load energy through the worst deployment interval. The design task is not to add a panel; it is to prove an energy-neutral ledger with enough autonomy for dark, cold, shaded, or quiet periods.

11.3 Energy Harvesting Design

Energy harvesting can extend battery life or support energy-neutral operation, but it does not remove the need for a battery-life budget. The source is intermittent, the converter has losses, storage ages, and the IoT workload still consumes energy on its own schedule.

This chapter treats harvesting as a field design problem. Start by measuring what energy is actually available at the installation point. Then size conversion, storage, and workload policy so the device survives the worst interval, not the best afternoon.

11.4 Learning Objectives

By the end of this chapter, 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.
• Decide when MPPT or a simple converter is appropriate for a low-power node.
• Identify unrealistic indoor-solar, body-heat, and vibration assumptions.
• Specify field evidence needed before claiming perpetual operation.

11.5 Harvesting Architecture

A practical harvesting system has four coupled parts: source, converter, storage, and load policy.

This stage of harvesting architecture calls for evidence at both Photovoltaic and cells convert. In the visual at Figure 11.1, the two labels make the required comparison explicit.

Three labels carry the instructional work in the chart at Figure 11.1. Photovoltaic adds a distinct review condition; cells convert adds a distinct review condition; sunlight to DC adds a distinct review condition. Read together, they establish this specific relationship: Solar energy harvesting system architecture where a photovoltaic panel feeds an MPPT charge controller charging a rechargeable cell, then regulation delivers a stable rail to the IoT load. The ongoing harvesting architecture analysis should preserve all three.

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 switched-capacitor or boost converter may change gain ratio, switching frequency, or load connection as the capacitor voltage rises. The review evidence should therefore include cold-start threshold, input voltage range, converter quiescent current, storage-capacitor leakage, voltage before and after a radio burst, and recovery time after a low-harvest interval.

Checkpoint: Architecture Fit

Before choosing parts, name the source condition you measured, the converter start point, the storage autonomy target, and the load state that can be reduced when energy is scarce. A design that cannot fill those four slots is still a concept sketch, not a harvesting architecture.

11.10 Source Reality Map

Source type alone does not tell us how much energy reaches storage. Inspect Figure 11.2 to connect the photovoltaic surface itself to the irradiance, shading, and collection losses that must be measured before solar can enter the source-reality map.

In Figure 11.2, begin with the repeated blue photovoltaic cells, then notice the narrow metal collection fingers and brighter vertical busbars crossing them. 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. 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.

Harvesting technologies are not interchangeable. Each source has a different failure mode.

A hidden assumption between 100s uW and mW could overturn source reality map. The chart in Figure 11.3 brings that assumption into the review.

Use mW as the pivot in the diagram at Figure 11.3. Before it, 100s uW adds a distinct review condition; after it, 10s mW+ adds a distinct review condition, while the pivot itself adds a distinct review condition. The resulting structure is not decorative: No-panel energy harvesting source map comparing outdoor solar, indoor light, thermoelectric, vibration, and RF harvesting by power range and availability risk. It sets the usable limit for source reality map.

Checkpoint: Source Evidence

Do not compare sources by name alone. Outdoor solar needs worst-month harvest and autonomy evidence, indoor light needs an occupied and unoccupied lux or panel-output record, TEGs need measured delta-T across the module, vibration needs a mounted acceleration spectrum, and RF needs received-power and duty-cycle evidence near the intended source.

11.11 Supply-Side Harvesting Examples

The demand-side answer to an energy problem is still the first answer: reduce waste, sleep aggressively, and spend computation, communication, sensing, decisions, and control only when they produce useful work. Supply-side harvesting adds a second question: is there cheap, clean, available energy at the deployment point when the node needs it?

Treat “create more energy” as a source audit, not a slogan. A hand crank or emergency radio may produce tens of watts per kilogram while a person is turning it; a shake flashlight proves magnetic induction but not continuous service; a shower-head turbine may have only milliwatts while water is flowing; a roadway harvester may claim kilowatts per car but has civil-work, traffic, and maintenance costs. The common review move is to convert each example into a measured source profile, then compare it with the load and storage ledger.

This is why the source map above is paired with the field record below. The holy grail is not simply harvesting more energy than the node needs on a good moment; it is closing the ledger across the worst interval without hiding availability, conversion, storage, or maintenance costs.

11.12 Energy-Neutral Ledger

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

Before the energy-neutral ledger argument advances, use Figure 11.4 to reconcile Supply over the window with Harvested input. The diagram turns that reconciliation into something the team can inspect.

At Supply over the window, Figure 11.4 establishes the starting condition. The visual next names Harvested input to show where the review establishes the starting condition; accounting window finally adds a distinct review condition. Those concrete stages express the following claim: No-panel energy-neutral ledger balancing supply (harvested input minus conversion loss) against demand (load energy, storage loss, and reserve) over the worst interval, such as a winter week or machine shutdown period. They keep energy-neutral ledger tied to observable evidence.

Use this accounting form:

• Harvested input: measured source energy during the accounting window.
• Conversion loss: rectifier, boost, MPPT, charge controller, and regulator loss.
• Load energy: sleep, sensing, compute, radio, retries, and maintenance states.
• Storage loss: self-discharge, leakage, depth-of-discharge limit, aging, and temperature derating.
• Reserve: the energy needed to survive the defined low-harvest interval.

Energy-neutral operation means:

$E_{\text{harvested}} \times \eta_{\text{conversion}} \ge E_{\text{load}} + E_{\text{storage loss}}$

That inequality must hold over the worst relevant interval, such as winter week, weekend lights-off interval, machine shutdown period, or cold outdoor deployment window.

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. 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. With two harvest hours and 80% charger efficiency, the source must supply at least 4.19 mW during those hours.

Math Bridge · guided foundationsHow large must the store be when the harvester goes quiet?Let Battery Bruno turn the chapter's state ledger into storage, panel power, and indoor-area requirements. Learn the maths

11.13 Communication Under Random Energy Dynamics

A conventional battery-powered radio often starts from a fixed transmit-power budget. A harvesting transmitter has a different problem: energy arrives over time, may be random, and is often known only when the device has already harvested it. The communication design must therefore respect both the channel and the storage state.

For a simple slot model, let E_t be the harvested energy arrival, B_t the stored energy available before transmission, and X_t the transmitted signal in that slot. The transmitter can spend only what is in the store:

$|X_t|^2 \le B_t$

After the slot, the next storage state is bounded by the storage size:

$B_{t+1} = \min(B_t - |X_t|^2 + E_{t+1}, B_{\max})$

This is the technical version of the ledger above. Source variability changes when the node can transmit, not only how much energy it uses per day. If a gateway or receiver does not know the harvest process, the protocol should tolerate deferred reports, shorter payloads, local buffering, or explicit low-energy status instead of treating silence as only a link failure.

In the ideal limit where storage is effectively unlimited, the long-run capacity of an additive white Gaussian noise channel can be treated like the classical Shannon channel with average transmit power equal to the average harvesting rate:

$C = W \log_2\left(1 + \frac{\mathbb{E}[E_t]}{N_0 W}\right)$

Finite storage changes the question. If B_{\max} is larger than the largest likely energy arrival, the store behaves close to the average-power case above. If B_{\max} clips arrivals, the useful transmit budget is closer to the average of min(E_t, B_{\max}) because excess harvest is spilled instead of saved. Correlated arrivals matter as much as the mean: a long low-harvest coherence interval can drain the store even when the long-run average looks acceptable, while clustered high-harvest intervals may be wasted if the store is already full.

The engineering lesson is narrow but useful: average harvested energy determines the theoretical link budget, while finite storage, cold-start behavior, and causal knowledge of E_t determine the field schedule. Use ordinary communication and coding techniques only after the measured harvest average closes; then add a storage-aware policy for the bad intervals. A good review states which regime the node is in, whether increasing B_{\max} is still useful, and whether the better fix is a larger store, a lower payload rate, or a different transmit-power policy.

Remotely powered communication adds one more coordination boundary. A charger may transfer energy at a fixed rate, observe only past channel outputs, observe intended transmitter inputs, or share richer side information about the transmitter’s battery state. More side information can let the charger time energy transfer closer to the moments when the transmitter needs it, but it is not free: the design must account for observation cost, control latency, regulatory power limits, and what the receiver should infer from silence. Record whether the transmitter adapts coding or modulation to instantaneous battery level and whether the gateway can distinguish low-energy silence from an ordinary link failure.

11.14 Worked Example: Outdoor Solar Sensor

Scenario: A low-power environmental node reports once per hour. A field measurement shows:

• Sleep baseline: 18 uA for almost the full hour.
• Sensing and compute: 6 mA for 3 seconds each hour.
• Radio transaction: 110 mA for 2 seconds each hour.
• System voltage: 3.3 V.

Daily load estimate:

• Sleep: 0.018 mA x 24 h = 0.432 mAh/day.
• Sensing: 6 mA x 3 s x 24 / 3600 = 0.120 mAh/day.
• Radio: 110 mA x 2 s x 24 / 3600 = 1.467 mAh/day.
• Total: about 2.02 mAh/day.
• Energy: 2.02 mAh x 3.3 V = 6.7 mWh/day.

Storage for 7 low-harvest days:

• Load over 7 days: 6.7 x 7 = 46.9 mWh.
• With 80% usable depth and 30% reserve: 46.9 / 0.8 / 0.7 = 83.8 mWh.
• At 3.2 V nominal storage, this is about 26 mAh; choose a larger standard cell after temperature and aging review.

Panel sizing:

• If the worst useful winter harvest window is 2 hours per day, and charger efficiency is 80%, the panel must deliver at least 6.7 / (2 x 0.8) = 4.2 mW average during those useful hours.
• Add site margin for angle, dirt, shading, seasonal uncertainty, and battery recharge after a dark period.

Design decision: The first sizing result is small because the node is already low-power. 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.

11.15 MPPT and Converter Gates

MPPT is not automatically better for every small node. It helps when the source has a moving maximum-power point and the controller overhead is small compared with harvested power.

Avoid component tables that imply a part is universally correct. The right converter depends on source voltage, source impedance, cold-start requirement, storage chemistry, leakage budget, and load bursts.

11.16 Storage Design

The photographs below make supercapacitor a physical comparison: look for changes in package, exposed interfaces, mounting, scale, and service access before treating the forms as interchangeable.

Read across the forms as engineering evidence. They share a capability name, but packaging and installation change the electrical, mechanical, environmental, and maintenance constraints.

Storage must handle two different jobs: long gaps between harvest events and short bursts from the load.

Storage must absorb the harvester’s irregular supply and still serve the load’s pulses. Inspect the photograph in Figure 11.5 before choosing a store because “a supercapacitor” spans very different capacitance, voltage, terminal, and packaging classes.

Scan the image in Figure 11.5 from the small cylindrical cells marked 3.0 V and 3000 F toward the boxed modules with heavy red and black terminals. The printed ratings describe individual parts, while the busbars, housings, and terminals reveal what changes when cells become a higher-energy assembly. This range reinforces the storage argument: pulse capability does not prove long gap autonomy, so usable voltage swing, leakage, balancing, and conversion losses still belong in the ledger.

For a burst buffer, the usable capacitor energy is:

$E = \frac{1}{2} C \left(V_{\text{high}}^2 - V_{\text{low}}^2\right)$

Use that equation only after checking equivalent series resistance, leakage, and the minimum voltage accepted by the regulator and load.

11.20 Continue to Part 2

Continue with Energy Harvesting: Source Reality and Field Proof.