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November 14[edit]

Electric/electronic device getting charged with static electricity[edit]

Why some non-earthed devices get charged with static electricity (and others not)?--Doroletho (talk) 00:21, 14 November 2018 (UTC)[reply]

It depends where they and whatever touches them are on the Triboelectric series. --Guy Macon (talk) 02:13, 14 November 2018 (UTC)[reply]

Aerothermal heating[edit]

I was expecting to find something else in the link above. It actually forwards to Aerodynamic heating (heating of a solid body produced by its high-speed passage through air). I was interested in a purported home heating/cooling system, that paired with a heat-pump, could harvest energy out of thin air (literally). That is, it can recover energy from air around the house. Supposedly, it could save up to 2/3 of power spent heating or cooling. Is this a viable heating and aircon system? Or is it an 'air-con' scheme? Is it possible to obtain energy from air (which has some energy in it, if not at absolute zero)? Or is it like the water powered car (that converts water into hydrogen with a battery, just to burn it in a fuel cell). I don't want to link to the companies offering it, but 'Air Source Heat Pumps' or 'Aerothermal heat pumps'. There's a Air source heat pumps article, but it is in chronically need of references and a more scientific background. --Doroletho (talk) 00:48, 14 November 2018 (UTC)[reply]

Your statement "air (which has some energy in it, if not at absolute zero)" is factually incorrect. You need a temperature difference to extract any energy from the air. See Heat engine.

A heat pump is simply a clever way of using less electricity to heat your home. A normal electric heater gives you one watt of heating for one watt of electricity. A heat pump is basically an air conditioner that is trying to cool the great outdoors and which has the cooling fins inside your house instead of outside. Every watt you put into it gives you one watt of heat, just as in the electric heater, but it also cools down the cold side (which is outside) by pumping heat to the warm side (which is inside), so you get more heat for the same amount of electricity.

I agree that the articles on this are a mess, and a quick look showed me some things that simply are not true, like "By definition, all heat sources for a heat pump must be colder in temperature than the space to be heated." Whoever wrote that has no understanding of how heat pumps work. I am a bit busy at the moment, but in a week or so I will clean up the articles. --Guy Macon (talk) 02:39, 14 November 2018 (UTC)[reply]

The Volumetric heat capacity of Air is very, very low. See Heat_capacity#Table_of_specific_heat_capacities where Air is listed with 0.00121 and Water with 4.1796. You would have to compress gigantic volumes of air to heat the water for your central heating for a house. Heat pump systems that draw heat from a grid of tubes installed underground, in a riverbed and alike commonly use Ammonia (3.263) or similar suited chemicals for the heat extraction. --Kharon (talk) 05:09, 14 November 2018 (UTC)[reply]

Ground source heat pumps are one solution and they do often result in energy savings compared to air source, but Air source heat pumps are probably the most common by far. I've never heard of anyone using a ground source heat pump in NZ (I mean I'm sure there's someone), but (air source) heat pumps are becoming increasingly popular her in NZ with a lot of ads on TV etc, promotion by the government etc. Of course winters here are fairly mild, especially in Auckland. And also most homes have no central heating plus piped gas or other non electric heating is not so common, with wood burners (whether crappy old fireplaces or modern insert type ones) being the most common non electric [1]. According to this [2] air and water source heat pumps have 7x the installations of ground source in Germany and combined they have also just exceeded gas, for domestic heating. I find the water source bit a bit weird, I wonder if they actually mean air to water. Actually on that note, I should mention heat pump hot water systems are also gaining ground in NZ. Nil Einne (talk) 06:58, 14 November 2018 (UTC)[reply]

BTW, as for your volumetric heat capacity, I think you're overanalysing things you don't understand. Remember as Guy Macon already said, the Coefficient of performance for heat pumps, with reasonable outside temperatures is significant above 1. If you can heat a home with a 8kW electric heater (to give a random example) then you can do so with a heatpump for less, perhaps 4kW. Whether you're directly heating the air of the home, or heating water to distribute it through the house doesn't ultimately matter in terms of what you need. (Okay it can affect how well the heat is distributed and efficiencies but you choose whatever solution you find is best for cost-performance reasons.) The only reason you're heating water is to eat the air, occupants and furniture of the house. Nil Einne (talk) 07:10, 14 November 2018 (UTC)[reply]

I'd be pretty careful with that stuff, just because it's brand name doesn't mean it's any use. For example, here summer dewpoints are 15-25°C but there's never a shortage of 300-1000€ brand name swamp coolers on sale, despite the fact that in any heat wave this climate makes them pretty much glorified fans. 78.0.233.248 (talk) 05:30, 14 November 2018 (UTC)[reply]

The phrase "You would have to compress gigantic volumes of air" above shows a misunderstanding about how such systems work. We aren't talking about using air as the refrigerant, but rather using fins with a fan blowing across them to do the actual heating/cooling of the air. No compression needed.

Re: ground source vs. air source, ground source costs more to install and requires access to some ground (not practical for a 30th-floor apartment in NYC) but the heat pump doesn't have to work so hard; on a really hot day the ground is cooler than the air, and on a really cold day the ground is warmer than the air. These are just engineering details; the basic design is the same either way. --Guy Macon (talk) 08:46, 14 November 2018 (UTC)[reply]

You seem to be describing what is known in Australia as a reverse cycle air conditioner. In summer we blow heat out into the atmosphere and cool the house. In winter we cool the atmosphere and heat the house. The latter is actually more 'efficient', as the power used to drive the pump also heats the house. Guy's points are also well made. We also have air to water heat pumps that use the air as a reservoir of heat energy and warms the domestic hot water. Again the pump energy actually ends up where you want it. These ssytems all work whatever the temperatures involved, that is, you can use air at 20 deg C to heat water at 60 deg C. Greglocock (talk) 10:24, 14 November 2018 (UTC)[reply]

Has anyone ever figured out a way to make one of those things so it doesn't make the entire neighborhood sound like a fan room? Related: Does anyone in the world value the sound of an honest breeze? Wnt (talk) 12:33, 15 November 2018 (UTC)[reply]

We know how to make quiet fans with any desired capacity -- basically more volume of air at a lower velocity and a careful design to maximize laminar flow and minimize turbulance. for example, https://www.bigassfans.com/ makes fans that move a lot of air without making a lot of noise. What we don't know is how to make a quiet fan as cheap as the noisy fans are. --Guy Macon (talk) 15:04, 15 November 2018 (UTC)[reply]

We have an ASHP that is much quieter than our previous oil boiler. It's a significant benefit.--Phil Holmes (talk) 16:29, 15 November 2018 (UTC)[reply]

The fan on our ASHP is rated at 34 dB, so it's basically inaudible. The compressor, however, is a bit noisier than the gas boiler that it replaced. Mikenorton (talk) 18:59, 15 November 2018 (UTC)[reply]

Are you talking about the total noise produced, or the total noise as experienced from inside the offending house if every window is always kept closed and the machinery is put somewhere outside on the far side of a barrier? Wnt (talk) 01:45, 16 November 2018 (UTC)[reply]

The internal machinery is in the place that the old oil boiler was - a small room within our kitchen. The external machinery is just outside the kithen window. It is much quieter inside and somewhat quieter outside, standing next to the external untit.--Phil Holmes (talk) 10:24, 16 November 2018 (UTC)[reply]