I try to average about 150 watts for an 80 minute period on the rowing machine. I'm about 20% efficient, so I need to move about 600 watts out of my body. Humans are well designed for dumping heat and within a few minutes I'm sweating - really sweating. The basement averages about 65°F during the Summer so I only wear cutoffs and still need to go through more than a liter of water. By late November the temperature has fallen to 50°, I start the session in cutoffs and a tshirt, but within five minutes I'm too hot and the tshirt comes off. In Summer and Winter I'm still uncomfortably hot for several minutes after finishing - the glow of exercise:-)
Last week was chilly. It was 17° when I took out the recycling at six am immediately after exercising. My neighbor saw me and shook his head and returned to scraping the heavy frost from his windshield. I was wearing my cutoffs and tshirt - no coat or long pants. Why wasn't I freezing in the chilly morning air? What do we mean by thermal comfort and how do we sense it?
You probably have between 1.7 and 2.0 square meters - roughly the area of a car's hood.1 Embedded in the skin is a rich sensor network. We tend to think about touch and pain, but there are also about 160,000 thermal receptors. They don't measure the same temperature measured by thermometers, but are sensitive to dry bulb and radiant temperatures, local air velocity, humidity as well as your current metabolic output. Their output is used to regulate body temperature and are the basis for your sense of thermal comfort.
It has been known for decades that dry bulb temperature isn't a great proxy for thermal comfort, but we uses it because it is easy to measure and integrate into HVAC systems. The result has been inefficient spaces that aren't optimally comfortable. A thermostat isn't going to help much - most only measure and stabilize dry bulb temperature in small areas. You can break a home or building into multiple zones, but the several of the quantities needed to calculate thermal comfort are missing. A 'smart' thermostat may save some money by only tightly regulating temperature during specific times when you are around, but that isn't enough. Perhaps an improvement over your conventional thermostat if you're lazy, but hardly smart.
Years ago I lived in an old Sanford White house built around 1890. There wasn't any air conditioning and the furnace was ancient, but it was surprisingly comfortable. The It was very good at allowing you to control and stabilize mean radiant temperature - a measure that can have a larger impact on thermal comfort than dry bulb temperature. You could also control local wind speed through the use of well placed windows and ceiling fans.
Local weather stations measure what is important to thermal comfort are some way off - probably be part of a wearable or, more likely, network of wearables embedded in or very close to your clothing.2 This network would have to communicate with local devices - including your clothing. Smart fabrics and clothing designs might dynamically regulate thermal insulation properties - its clo, a kind of R value for clothing.3
You can take steps to be more comfortable and reduce your energy bill at the same time. Insulating the body is much more efficient than heating a full house. For cold winter days modern thermal underwear can be extremely effective and even comfortable. Make use of the fact that those 160,000 thermal sensors aren't evenly distributed - they're more concentrated on your your feet, ankles and calves, hands and wrists and neck, and face. Dress accordingly and use radiant heat as necessary rather than cranking the thermostat. Move around a bit rather than just sit - crank up your metabolic output. Minimize drafts - sealing air leaks is usually one of the most cost effective ways to lower your heating and air conditioning bills.
Air conditioning works by lowering the air temperature and sometimes humidity. The lower temperature increases your heat loss by convection and lower humidity by evaporation of skin moisture - even when you're not sweating up a storm. Fans work by increasing the air velocity across your skin increasing heat loss by convection and evaporation (if the air isn't too humid or hot). There is also passive or active radiant cooling, but let's stick with fans for the moment.
Moving air requires much less energy than refrigerating it. The cooling effect is immediate and can be local. Heat evaporation loss increases roughly as the square of air velocity, but thermal comfort is a bit more difficult to determine. Some rules of thumb are a one meter per second airflow can offset a 3°C (5.4°F) increase in indoor temperature and three meters per second is good for 7°C.4 There is a lot of experimentation you can do. I use a small reasonably efficient personal fan installed under my desk that creates an airflow towards my chest and head.
Computational fluid dynamics is being used to design quieter and more efficient fan blades and more efficient electric motors are being used. Some ceiling mounted fans can gimbal and deliver airflow to where it is needed. While fans have a fundamental limit of not being effective as air temperature approaches your skin temperature (about 95°), they are generally underused and a good deal of design cleverness remains to be discovered. Last Summer I started using a fan during my exercise sessions - it made a dramatic difference in my comfort as well as ability to maintain my power output level.
Much can be done by thinking differently about design and biology - and even rediscovering the past. But remember that no matter what you do, it makes sense to measure what you are trying to control.
1 Knowing a person's skin area is useful in medicine and sports science. A variety of techniques are used, but a quick estimate, good to a few percent on people with average builds is the Mosteller formula.
body surface area (m2) = (height(cm) * weight(kg)/3600)0.5
The formula loses accuracy for those with very low or high body fat percentages and those who have lost a lot of weight.
2 ARPA-E has announced a program designed to investigate micro thermal regulation as well as thermal comfort regulation.
3 clo is a measure for clothing insulation and has the same units as R values (°K-m2/watts). 1 clo = 0.86R. A tshirt has a clo of about 0.1, a flannel shirt about 0.35, a normal down coat is about 3.
4 This is a rough rule of thumb. The effect is more pronounced at lower humidities and less in wetter air. Fans make a lot of sense in the desert Southwest. Quite a bit of experimental work has been done correlating thermal comfort with as a function of fan position and wind speed. This paper from the Research Division of the California Air Resources Board is an excellent resource.
The excess of Thanksgiving is over, so a simple and healthy soup seemed in order. It is great with good bread. I think it improves if left overnight in the refrigerator, but if you have several enthusiastic eaters you may not be able to test my conjecture.
° 3 15oz cans of chickpeas,
°3 tablespoons olive oil
° 2 large yellow onions, coarsely chopped
° 4 garlic cloves, chopped
° sprig thyme
° 1/2 cup white wine
° 4 cups of vegetable broth (no salt)
° heat the oil in a large pot over medium heat. Cook onions, garlic and thyme until the onions are soft
° add chickpeas and wine, bring to a rapid simmer and cook for a few minutes to reduce the wine by about half.
° add the broth and crank the heat to a boil. Reduce the heat to a simmer and cover for about a half hour.
° purée with a food processor or, better yet, an immersion blender. Salt to taste.