Energy & Power · Study deck

Energy Harvesting: Source Reality and Field Proof

A bench ledger balances and the storage calculation looks safe, but the field source is weaker and less regular than its peak rating.

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:

Explain: A source graphic associates these same $C$ and $V_f$ labels with 192 nJ, but those numbers do not satisfy $\tfrac12CV^2$ ; the chapter keeps the physically consistent derivation instead of propagating that mismatch.
Explain: An eight-ratio design can therefore cover a 0.6–2.4 V source while regulating a roughly 1.2 V load more efficiently than one fixed ratio across the whole range.
Explain: This conclusion depends on the assumptions above; finite horizon, leakage, conversion loss, non-stationary weather, or strict delay limits can all make the arrival shape matter again.

Major section

TEGs Need Delta-T Across the Module

A hot pipe or warm enclosure is not enough.

Thermoelectric output depends on the temperature difference across the module after thermal contact resistance and heatsink limits are included.

A TEG module only produces useful power while its two faces stay at different temperatures; clamping both faces into the same warm enclosure removes the gradient that drives it. Photo: Gerardtv, CC BY-SA 3.0

Major section

Indoor Is A Thousand Times Weaker, And Peak Is Not Average

The orders-of-magnitude descent explains why a panel that is generous outdoors can be inadequate indoors.

The scale also separates peak irradiance from daily harvest, so sizing must use the worst credible light schedule and storage gap.

The useful solar-harvesting region moves by orders of magnitude between outdoor sun and indoor lighting. The same load that is easy outdoors can require page-sized cell area indoors once peak-sun-hours and derating are counted.

Major section

Energy-Arrival And Random-Access Calculator

An energy-harvested radio cannot schedule from average power alone.

Energy arrives unevenly, storage is finite, sleep electronics consume part of every arrival, and a random-access collision can force another expensive transmission.
Its slotted random-access model assumes each of the contending nodes transmits independently with probability $p$ in a slot, giving this node a success probability of $p(1-p)^{n-1}$ .
Observe why reducing traffic can improve both collision probability and energy survival.

Major section

Energy-Harvesting AWGN Capacity

The finite-store ledger makes that clipping mechanism inspectable slot by slot.

The inequality is the causality rule; the min is the overflow rule.
Energy arriving when the store is full is clipped and cannot be recovered later.
An infinite store can absorb high-arrival slots and release their energy during low-arrival slots.

Infinite and finite energy-harvesting batteries are compared through usable power and AWGN capacity expressions.

Major section

Energy-Harvesting AWGN Capacity (continued)

This conclusion depends on the assumptions above; finite horizon, leakage, conversion loss, non-stationary weather, or strict delay limits can all make the arrival shape matter again.

Finite storage changes the problem.
so the usable mean can fall below the harvested mean.
Actual overflow depends on the current buffer and power-control policy, and the logarithm is concave, so bursty power allocation does not behave like constant average power.

Major section

Series-Parallel Harvesting Converter

A switched-capacitor converter changes voltage by reconfiguring flying capacitors between charge and transfer phases.

A family of switch configurations provides discrete conversion ratios.
An eight-ratio design can therefore cover a 0.6–2.4 V source while regulating a roughly 1.2 V load more efficiently than one fixed ratio across the whole range.

Weak vibration, light, thermal, or RF energy is rectified, stored, converted by switched-capacitor stages, controlled by sensing, and released through power-good qualification.

Major section

Step-Charging Derivation

Connecting an ideal voltage source directly to an initially discharged capacitor stores $\tfrac12CV^2$ in the capacitor and dissipates the same amount in the series path.

If the voltage rises through intermediate levels $V_0,V_1,\ldots,V_N=V_f$ , the redistribution loss becomes.
A source graphic associates these same $C$ and $V_f$ labels with 192 nJ, but those numbers do not satisfy $\tfrac12CV^2$ ; the chapter keeps the physically consistent derivation instead of propagating that mismatch.
Real step charging also spends energy in switch gates, clock generation, leakage, and finite resistance, so ever-smaller steps eventually lose to control overhead.

Deck summary

Key takeaways

A hot pipe or warm enclosure is not enough.

The orders-of-magnitude descent explains why a panel that is generous outdoors can be inadequate indoors.
An energy-harvested radio cannot schedule from average power alone.
The finite-store ledger makes that clipping mechanism inspectable slot by slot.
This conclusion depends on the assumptions above; finite horizon, leakage, conversion loss, non-stationary weather, or strict delay limits can all make the arrival shape matter again.

Retrieval practice

Recall check 1 of 4

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

Q1A node consumes 6 mWh per day. A measured solar source provides 10 mWh on a winter day before conversion, and the charger/regulator path is 75 percent efficient. What is the best review conclusion?

AUsable harvest exceeds load; verify storage for bad intervals.

BThe system fails because 10 mWh is less than twice the daily load

CThe system is proven perpetual because the daily average is positive

DThe charger efficiency can be ignored because the source is renewable

Show answer

Answer: A Usable harvest is 7.5 mWh/day, which exceeds the 6 mWh/day load. That supports the design, but it does not prove survival through low-harvest intervals or storage recovery.

Retrieval practice

Recall check 2 of 4

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

Q2What buffers the gap between harvested energy and load demand under voltage and leakage limits?

AStorage

BAmbient source

CConverter

DLoad policy

Show answer

Answer: A Capacity, leakage, and voltage limits buffer the gap between harvest and demand.

Q3What does energy-neutral operation require of a harvesting node?

APeak harvested power exceeds peak load current at one instant.

BThe device never sleeps, so it always uses the harvested power immediately.

CThe battery is removed entirely and no storage is used.

DAverage harvested power, after losses, at least equals the average load.

Show answer

Answer: D Energy-neutral means the daily harvest at least matches the daily consumption, and because the source is intermittent, storage must bridge nights, cloudy spells, or still periods.

Retrieval practice

Recall check 3 of 4

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

Q4A node needs 15.8 mWh per day. Using 2 peak-sun-hours and a derating of 0.7, what panel peak power makes it energy-neutral?

AAbout 11.3 mW, dividing 15.8 mWh by 2 h times 0.7.

BAbout 0.66 mW, using daily average load as panel peak.

CAbout 7.9 mW, omitting the 0.7 harvest derating.

DAbout 190 mW, applying the load over only two hours.

Show answer

Answer: A The panel must supply the daily energy within the effective harvest window: 15.8 / (2 x 0.7) = 11.3 mW peak. At 15 mW/cm2 that is under one square centimeter of outdoor cell.

Retrieval practice

Recall check 4 of 4

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

Q5A solar node proven outdoors with a 1 cm2 cell is moved indoors and immediately starves. What is the fundamental reason?

AIndoor light has the wrong color and solar cells cannot use it at all.

BThe load must have increased when the device moved.

CMoving indoors increased self-discharge of the panel.

DIndoor light delivers about 10 uW/cm2 versus about 15 mW/cm2 outdoors.

Show answer

Answer: D The output density collapses by about three orders of magnitude indoors.

Print reference

Answers 1 of 2

Answer key.

A · Usable harvest is 7.5 mWh/day, which exceeds the 6 mWh/day load. That supports the design, but it does not prove survival through low-harvest intervals or storage recovery.
A · Capacity, leakage, and voltage limits buffer the gap between harvest and demand.
D · Energy-neutral means the daily harvest at least matches the daily consumption, and because the source is intermittent, storage must bridge nights, cloudy spells, or still periods.

Print reference

Answers 2 of 2

Answer key.

A · The panel must supply the daily energy within the effective harvest window: 15.8 / (2 x 0.7) = 11.3 mW peak. At 15 mW/cm2 that is under one square centimeter of outdoor cell.
D · The output density collapses by about three orders of magnitude indoors.