Human Cogeneration: Generating your own Heat and Electricity

Published on 2023-01-21 by Jim Gregory
reading time: ~8 minutes

Most fossil-fueled power plants convert less than 40% of the energy they consume into electricity. The remaining 60% is lost as heat. To increase efficiency, some power plants capture a portion of this waste heat and put it to use elsewhere. This combined generation of electricity and useful heat is called cogeneration or combined heat and power (CHP).

A typical example is a power plant teamed with a district heating system, which I've illustrated in the top half of the figure above. Here, steam or hot water is extracted from the power plant's waste heat and piped through the radiators and heat exchangers in nearby buildings, providing them with heat and domestic hot water.

Cogeneration also occurs whenever you pedal a generator in a cool environment. The human body is only about 20% efficient at converting food into useful work. The remaining 80% is given off as heat, in a process known as exercise activity thermogenesis (EAT). This EAT-derived heat both makes you warm and helps heat the area around you, too, as shown in the bottom half of the figure.

I call this simultaneous production of heat and electricity while pedaling a generator human cogeneration. It is uniquely differnt from conventional cogeneration, and offers some energy-saving opportunities that are worth exploring.

Human Cogeneration vs Conventional Cogeneration

While human cogeneration is less efficient at converting fuel to electricity as conventional cogeneration, it's significantly more efficient at making you warm.

In a CHP district heating system, the heat primarily warms buildings, not people. People become warm only because the building and surrounding air is warm. The total energy needed to keep each building warm is much larger than that actually needed to warm the bodies of the people inside.

What's more, the heat is distributed through a complex system of pumps, heat exchangers, pipes, and radiators, all of which require energy.

Human cogeneration, by contrast, is much simpler: your body warms itself, as a consequence of EAT. Since there's no need to warm your surroundings to warm your body, the energy needed to make you comfortable is drastically less. Your excess body heat helps warm your surroundings (just like a radiator in a CHP district heating system!), but this only happens after you become warm first.

How Human Cogeneration Can Save Energy

Replacing a Space Heater

If you perform sedentary work in a poorly heated space, chances are you use an electric space heater to keep comfortable. These heaters typically draw 1500 W. If the space heater's duty cycle is 100% while you work--i.e., the heating elements never turn off--then the space heater consumes 1500 Wh (1.5 kWh) every hour of your workday.

EAT from pedaling a human-powered generator while you work can generate enough heat to keep you comfortable, completely eliminating the need for a space heater. (I typically work in shorts and a T-shirt--and still need a fan to stay cool!) This reduces your energy consumption by 1.5 kWh each hour you work. If you work a normal 8 hour day, this 12 kWh/day savings can make a measurable difference in your overall electricity consumption.

Prolonging a Thermostat Setback

During the heating season, our thermostat automatically sets back our indoor temperature 5°F / 3°C every night to save energy. I typically get up a couple of hours before my wife to get some work done in my human-powered office. Since pedaling the generator keeps me warm while I work, I let the thermostat maintain the setback temperature until both of us are up.

Using standard building science energy calculations, I estimate prolonging our setback saves us 380 Wh of heating energy per day for every additional hour of a 5°F / 3°C setback.

Greater setback amounts yield more energy savings. A 10°F / 6°C setback saves twice the energy of a 5°F / 3°C setback.

It's worth noting that the energy savings depend only on the setback amount, not the outdoor temperature. A setback on a cool spring evening saves the same amount of energy as the same setback on a cold winter night, provided the temperature indoors always remains above the temperature outdoors.

It's also noteworthy that this energy-saving method isn't just applicable to early-risers like me. If you're a night owl, you can use it, too. Just start the setback period to begin when everyone else in your household goes to bed.

Reducing Heat Load

A person generating 60 W of electricity using a pedal generator with an overall efficiency of 70% expends 60 / 0.7 = 86 W. As I noted above, a cyclist's overall efficiency is around 20%, so the person's total energy output (electricity + heat) would be 86 W / 0.2 = 430 W. Of this 430 W, 60 W is usable electricity. The remaining 430 - 60 = 370 W is given off as heat, either from the personal's body or the components of the generator system.

However, all the electricity eventually turns into heat, too, meaning all 430 W contributes to heating the surrounding space.

How much a difference this additional heat makes in your heating bill depends on the outside temperature, the size of your home, and how well insulated it is. For our leaky, 100 year old, 900 sq ft / 82 sq m home, I estimate 430 W of human cogeneration reduces our hourly heat load by 12% when the outdoor temperature is 32°F / 0°C and 16% when it's 50°F / 10°C. If our house was built to more modern building standards, those energy savings would be 19% and 37%, respectively.

Since I only pedal a fraction of the day, the energy saved over a day's time is much less. Using our monthly heating bill data and my pedal generator's daily output log, I calculated the fraction of heat my pedal generator supplied to heat our house. The results are shown in the table below.

¹ - Assumes 96% efficient forced-air gas furnace. We use negligible amounts of hot water and cooking (our summer monthly gas consumption is <1 therms) so all the gas we consume essentially heats our house.

² - Assumes 20% human power conversion and 70% electricity conversion efficiencies

Drawbacks of Human Cogeneration

Human cogeneration has three weaknesses. First, heat and electricity are only produced while you pedal. If you stop pedaling, heat and electricity production stops, too. To prolong the benefits from cogeneration, your workspace should be well-insulated to prevent losing the heat generated, and your generator system should have a battery to store any excess electricity you generate, so you can use it later when you're not pedaling.

Second, human cogeneration can be too effective at making you warm. You can break out in a sweat if you're pedaling hard and dressed too warm or don't have enough air flowing across your skin to keep you cool. As this perspiration evaporates from your skin after you stop, this can leave you feeling paradoxically too cold afterward. To prevent this from happening, dress appropriately (e.g., shorts and a T-shirt if you're pedaling hard) and use a cooling fan (or two!) to maximize heat transfer from your skin.

Lastly, when it's hot outside, the heat generated from human cogeneration goes from being a benefit to a liability. Conventional CHP systems can still operate under these conditions, heating water for domestic hot water or cooling air in special air conditioners.

Human cogeneration can't provide these benefits, but it can reduce your energy consumption by making some low-energy activities more palatable. An hour of pedaling, for instance, makes a cool glass of tap water more appealing than a cup of hot coffee, and a cold shower over a hot one.

Conclusion

Human cogeneration is extraordinarily effective at keeping you warm, because it generates internal heat inside your body as opposed from receiving heat from your surroundings. The activity can reduce your household's energy consumption by eliminating the need for a workspace space heater, extending an overnight thermostat setback, and/or directly supplementing the heat supplied to your home. I estimate the heat produced from human cogeneration reduces our home heating energy consumption by approximately 2% in the coldest months of the year and up to 25% in more temperate months.

The heat produced from human cogeneration is ephemeral and not always desirable. If you pedal hard enough, it can make you break out in sweat. To get the most benefits from human cogeneration, you must properly manage this heat.