Buildings with lots of equipment, like machine shops, hospitals and server farms, put out lots of waste heat. Which is counteracted with energy-sucking AC. Is there any way that the heat could be harnessed instead of just dumped to the outside? There are technologies that will charge a cell phone with the user’s body heat. Could something similar be scaled up, so that large buildings can recover energy from the heat their equipment puts out?
Waste heat is a very poor source of energy. Heat engines work on temperature differentials, and waste heat is not hot enough to be very efficient.
We can certainly harness heat differentials…even small ones. Here is Bill Nye the Science Guy harnessing energy from a hot cup of water and a cold cup of water with a Stirling Engine.
So yeah, you can harness that energy but making use of it is something else. Can you get the engine in the above video to do useful work? Maybe but probably not much.
Generally what decides these things is cost. If you cannot make the engine pay for itself why build it? Till now energy has been pretty cheap so no need to dive deep for marginal sources.
That said I have wondered why the automatic faucets in my building seem to require batteries to operate. Can’t they use the flow of water to turn a little generator to charge a capacitor that runs the very low power IR beam and operate the valve? It might be more expensive to install but seems overall less expensive than having to put batteries in.
Along those lines I have also wondered why elevators don’t do the same thing. When going down why not let gravity provide the energy and charge a capacitor that can be used to help lift it back up? I know there has to be a net energy loss and you need external power to the system but it should mitigate power costs. Akin to regenerative braking in cars. We do that there so why not in elevators or escalators (the down escalator helping to run the up escalator)?
Cars loose a ton of heat, 50 years ago I assumed by now it would be harnessed and used somehow.
Internal combustion engines produce waste heat. In automobiles, we capture this waste heat to warm up the cabin in cold weather. A similar concept could apply to an office building. You could have an electrical generator in the basement, providing electricity to the building, and harness the waste heat to warm up the building in cold weather. I read this concept about 20 years ago as “co-generation”.
There is a technology called district heating that harnesses waste heat from industrial facilities such as power stations or factories and pumps it through hot water or steam pipes to heat other buildings. They’re often used in large industrial complexes where a company uses heat from its factory to heat, e.g., its office building next door; but there are also extensive systems that supply entire cities. Many of them were built, in particular, in the Communist bloc (and many of these systems are still around in Eastern Europe), but also exist in other countries.
Usually the payback period for installing waste heat energy recovery is too long or does not come out positive in any time frame due to added maintenance cost of the system. As noted by others, it can be done, even at a profit, in some large scale waste heat producing facilities.
If the mindset, laws, building codes, etc… were changed appropriately. More efficiently sized and utilized systems could be used. A city block could have an area devoted to a heating and cooling system for that block. The users paying to use and maintain it. That facility could be linked to the adjacent blocks systems in a load balancing scheme. Fewer, larger systems may allow cost effective, lost energy recovery systems to work. The concept exists and works in several places.
On the edges of my city, there are 2 or 3 large natural gas fired electricity generation plants. With nothing around them. There are ideas to place businesses that can use that waste heat near them. Unfortunately, those businesses are borderline profitable here in the further north. ( green house facilities ) Ideally, they should have demolished a few industrial areas that have been surrounded by the rest of the city. Built the plants there. Piped the waste heat to provide building heat in our very cold, long, winters. Relocating those industrial areas would allow us to remove railway infrastructure crisscrossing and messing up the city. Naturally divert heavy truck traffic out of our city streets. Free up a lot of currently wasted space industrial area for more dense, efficient, tax revenue better, use.
The direct and ripple effect savings in energy, money, time, infrastructure use and wear, is massive. By just placing a resource in the right place, at the right scale.
I know it aggravates me that during the cooling season I have an air conditioner rejecting heat to the outside, and not 12 feet away on the other side of a wall I have a water heater keeping a tank of water at 140 deg F, but the capital investment for getting that waste heat from the air conditioner to the water tank is prohibitive. The savings won’t pay for the hardware.
There’s a continuum from active cooling, through passive cooling, to extracting energy from waste heat. If you’re doing active cooling it’s likely because passive cooling isn’t enough, or it isn’t cost effective to install the necessary technology, so moving beyond even that to extract energy from the heat flow is very unlikely.
That said in areas like here in Norway where it swings between a need for cooling and a need for heat solutions that dump heat in the bedrock in summer and extract heat from bedrock in winter are a thing. So the energy is sort of recovered, to supply heat at a later date.
All buildings of any significant size are heated mostly by their waste heat. In most places, you need to run air conditioning even in the winter to keep the building comfortable.
My understanding of the faucet situation is that the flywheel and additional circuitry and storage system (it still needs a battery, I don’t believe a capacitor will do) just makes it bulkier, more expensive, and a lot more prone to failure. The flywheel itself would need a watertight seal around its turning shaft, which if it leaks would leak into the electronics, plus the flywheel itself could break and cause problems.
As for elevators, in high rise buildings that use roped elevators, there’s already a counterweight, so the power used to go up is actually pretty close to the amount used to descend. It’s more momentum than gravity, though I believe the counterweight isn’t usually as heavy as the cab, so it’s not entirely balanced. Still, those motors can draw anywhere from 4 to 40kW, the higher being used for skyscrapers that need to accelerate the elevator significantly more. There are regenerative options out there, but I don’t think they store the energy, they just dump it back into the grid.
Shorter hydraulic elevators which have a piston that goes into the ground use about 3x as much power as a roped one, like 15 to 60kW even for just a few floors, as it’s pumping oil into the piston to raise it, but little or none to go down. If the oil in the system was sent through the pump backwards when the elevator descends, it would act as a generator, but my guess is there would be way too much resistance, leading to a very slow elevator, and then it would require a massive bank of batteries that could provide the significant amperage required to push the elevator up. That’s a battery bank about equivalent to what’s in a Tesla, plus a built-in supercharger. That would add low 5-figures to the cost, which is unlikely to be recouped anytime soon, especially with all the additional equipment that requires maintenance.
Here’s the Wikipedia article on Cogeneration. Not exactly what the OP is asking, but it is a technology under development that seeks the same benefits.
A regional grocery chain, Fareway, harnesses the heat from their refridgeration compressors to heat their store in the winter.
Not sure is actually economical though
All grocery stores do that. In fact, you’d have to go considerably out of your way to avoid doing that. The only difference is that Fareway has figured out that they can brag about it.
I don’t think so. Most grocery stores have their condenser units for the frozen and refrigerated cases on the roof. Some of the smaller units scattered around random parts of the store might be self-contained, but those are the exception. The issue is that if they’re dumping heat from the frozen food and produce aisles into the store to heat it in the winter, that means they need an air conditioner that’s twice as big (if not more) to cool it in the summer, since they’re pumping the heat out twice, losing efficiency each time. Also in the winter they can run the refrigerant systems at lower head pressure and get much better efficiency out of them because of the lower condensing temperature. Plus it’s not like you can eliminate the traditional heating system anyway, because if the frozen food cases go down your store also freezes, so there’s no upfront cost savings.
Thanks! All of that is really interesting.
Regarding the faucets I realize adding a little generator and battery/capacitor complicates things but I have had fish pumps for my aquarium pumping water non-stop for years. Seems to me such things are pretty inexpensive, durable and reliable. That is not the same as a little generator but suggests to me such things can be durable.
Perhaps in the end the energy saved is less than the added cost over the lifetime of the product (add in maintenance costs for people replacing batteries). Probably is else they would do it I would guess.
Gavin’s Point dam in South Dakota is heated by the friction of the turbines. Additionally, the Mall of America has no central heating system.
Factories will sometimes use the hot water which comes from cooling the reactor to heat up the offices.
Maybe one of our posters in Sweden will be able to find better information on this than I can, because all my information is hearsay, but I understand that the city of Landskrona uses heat from the factories (among other sources) for its heating.
Some of my clients have processes which generate a lot of steam. Steam -> turbine -> electricity -> profit! The last step will depend on what local energy prices are, but for some of them that energy generation meant the difference between red and black.
Using waste heat to generate electricity is generally impractical, because whatever is generating the waste heat needs to get rid of that heat. If you attach some heat-trapping device to try to extract energy from the waste heat, you often end up impeding the flow of heat, and causing the machinery to overheat.
Your building already pays someone to clean and maintain the bathrooms, so the additional cost of having the janitors replace the battery once in a while is probably negligible.
Also, many newer automatic faucets and flushers are powered by little photovoltaic cells (“solar panels” except the are powered by ambient indoor light, not sunlight - example). Photovoltaic cells have no moving parts, so they tend to be cheaper and more reliable than water turbines. These things use very little power anyway - I believe the valve’s operation is aided by the water pressure.