“Man’s own metabolism is a renewable energy source which he can use freely as long as he exists. It is obvious therefore that it should be exploited for the heating of buildings.”
One defining criteria of a passivhaus is 10W/m² maximum power to maintain 20°C internally when it’s -10°C outside. Within a passivhaus, the heat contributed by the human body is just one of the many energy transactions that occur.
We eat, we heat. The amount of that heat can be quantified – given certain assumptions.
This tells us the body generates heat to compensate for heat lost to the environment via metabolic processes a.k.a. living. When it’s colder, our bodies make more heat to make us feel less cold. Putting some clothes on is a good idea.
It’s a good idea because more, thicker and fitted clothing ‘keeps the cold out’ by slowing the rate of heat loss. This site explains the following graph in more detail.
When it’s hot, evaporative cooling driven by sweat and air movement prevent the body from overheating and fewer, lighter, and looser-fitting clothes are a good idea. Around the world, vernacular architectures have developed as extensions of these principles and also keep people dry and safe. This is what makes them buildings and not clothing or fashion.
The ideal state of a Passivhaus is for energy losses to equal energy gains. The breakdown of that balance changes according to the season and the day, as people come and go and do whatever it is they do.
Variation is not only inevitable but desirable. Imagine you’re in your passivhaus about to sleep or have a nap. You and the world are in thermal equilibrium but still you pull a cover or throw over yourself. This suggests your body is more comfortable when it’s allowed to find its own equilibrium.
The delicate energy balance inside a passihaus is easily upset by people coming and going, opening the refrigerator door, cooking stuff, eating stuff and doing stuff and none of these inputs is constant. Human metabolic heat generation is the least understood, least quantifiable and least controllable of all those inputs. It’s not all bad though. At least it means we’re alive.
The rate of metabolic heat production fluctuates according to what we eat and the time of day. It can work with or against the passivhaus heat balance but, as a general rule, it helps that we tend to not want to eat cold things in winter or hot things in summer. It’s also convenient that our core body temperature falls at night to lower the point of energy balance.
To be honest, I was hoping to discover some research into “Optimising human diet-related metabolic heat generation in passivhaus environments” but no. Instead, in those dark corners of the web where sites have names like 10 Common Golf Swing Mistakes, I found much advice on how to lose weight by ‘boosting your metabolism’. In principle, this will generate metabolic heat as a by-product so, with that disclaimer …
7 High Metabolism Foods
- Garlic – we’re not surprised. We know garlic is good. The Romans did.
- Strawberries – apparently because they contain anthocyanins, but the science is sketchy.
- Green tea – we hear this a lot, but if it has any effect at all, it’s because of the caffeine. This has been studied in more detail, here for one.
- Beans – anything with fibre in is meant to make the body ‘work harder’ to digest it. This sounds logical but evidence to support it is thin on the ground.
- Hot peppers – as with garlic, we knew this. Capsaicin is responsible. The mechanism is disputed but the endorphin rush – that chilli high – is fact.
- Lean meats – again the science, such as it is, is that proteins take longer to digest.
- Water – water is necessary for many metabolic reactions but what we really want to know is if drinking excess water initiates further metabolic reactions.
It’s not much to go on, but it’s a start.
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The burning question is can we and our passivhaus work together by a diet of steak and chips? The answer is yes and no. Any heat we would generate from metabolising them would be more than negated by the amount of heat required to rustle up those proteins, fats and carbs. Cooking is all about artificial heat imbalances that can be sudden and intense or low yet prolonged. Cooking is not what you really want to be doing in passivhäuser.
Fortunately, steak can be delicious raw.
– as can some veg, but the list of ingredients for this Raw Potato Salad suggests potatoes aren’t one of them.
We don’t much like potatoes uncooked – or unaccompanied. Ketchup isn’t tricky to make but it does require several ingredients not always available.
It also involves heat expenditure. Unless we’re prepared to be raw foodists or pre-fire Paleos, we may as well just heat our passivhaus directly if we’re going to cook anything ‘gently over a low heat for 10 to 15 minutes until softened, stirring every so often.’
- Once we finally work out what’s best to eat in terms of metabolic heat production, we’ll need to deduct any energy added to make it palatable by cooking it.
- If we’re going get holistic about it, we’ll also have to to factor in the amount of energy required to produce those ingredients. It’s often claimed rearing livestock is a grossly inefficient use of land. Given the proportion of the world’s population that already subsists on grain, this is probably true.
- We also can’t ignore the energy used to get that food to us. It’s a contemporary British Christmas tradition to calculate the number of air miles clocked up by the ingredients for a traditional Christmas dinner.
And let’s not forget the energy required to preserve food between its delivery and consumption.
There are many ancient methods for the long-term preservation of food long but again, it’s a mixed bag. Salting meat or fish doesn’t require additional energy but smoking it does. As for preserving foodstuffs short-term, Australia’s Coolgardie Safe and Russia’s Khrushchev Refrigerator (Хрущевский холодильник) require zero energy input, the former performing better with a slight breeze and the latter performing better in winter. As you’d imagine.
Our food preservation method of choice is refrigeration, but it’s gentler on your passivhaus if you allow meats and vegetables to reach room temperature before any frying, boiling or roasting takes place. Even if we consider only the energy used for cooking, balancing that factor within the passivhaus environment will require superhuman levels of awareness and self-control we simply don’t have.
We’re going to have to invest in heat recovery systems to claw back some of the heat that would otherwise be exhausted to outside. It smooths things out a bit and we can feel easier about eating cooked food and showering with water not cold.
Heat recovery systems alone are insufficient, and simple thermostats that average out extremes rather without doing anything to reduce them are pointless. We need a computerised control system such as at the Princess Elisabeth Antarctica (zero-energy) Antarctic research station. The station is a mostly closed environmental system and its energy balance can be closely monitored and controlled.
This is an interesting and useful development because the allocation of energy involves a Degradation Of Lifestyle – i.e. the user might have to wait. This is something we find difficult to comprehend but, if there has to be energy demand management, it is a very intelligent and fair way of doing it.
Energy demand management is happening right now in the world in conflict-ridden places where electrical and/or petrol supply is irregular and could easily be part of our future should energy supply become intermittent or limited.
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I began this post wondering how the human metabolism could be better integrated into a passivhaus but – hands up – I’m no more enlightened. But if we were more enlightened about the heat our bodies contribute to our environments we might make better use of it and maybe even generate it more efficiently.
Using metabolic heat in buildings isn’t a new concept – it’s just something we’ve had no need to continue to be aware of. Both Europe and America have histories of housebarns using the metabolic heat of livestock as underfloor heating in winter.
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- Passivhaus Designer’s Manual
- Passivhaus Technical Requirements
- Avoid the hassle – fuel-up before heading home!