District heating to electricity - A thought experiment - Page 1
Forum section: electronics / general

author: tszaboo
author_rank: Super Contributor
timestamp: April 26, 2020, 03:18:02 pm
content: I have district heating in my house.It means that I dont have a boiler or a furnace in the house, instead, there is connection to a network, where hot water circulates the whole year. It provides heating, by directly circulating this water in the radiators. It provides hot water for shower through a heat exchanger. Hot water temperature is about 70C, the cold water is (for example) 15C.The engineer in me just clicked, there is an intentional transfer of heat, from water to water happening. That can be used to generate electricity. Thermoelectric effect. I have to calculate if this is harvestable, how much it would be, how much the equipment would cost, and the ROI of such system.

author: tszaboo
author_rank: Super Contributor
timestamp: April 26, 2020, 03:18:51 pm
content: So how much energy is being transferred, for example when taking a shower. Taking a shower uses about 60L of water (average person) and the temperature of water is 38C. Some of you will outrage, that this is too high, maybe. It is marked on my thermostatic water mixer, so I'm going with that. We need ballpark figures anyway.Now, I think it doesn't matter if it heats up 60L of water from 15 to 38, or it heats up 30L of water higher and then mixes it with cold water. The energy is the same. The waters specific heat is 4.2 kJ/kg, we heat up 60kg of water by 23 degrees C. That is 5796 kJ, or as engineers call it, 1.6 kWh.TEG are in the range of 5 to 10% efficient with ideal conditions. I'm not going to investigate an existing TEG to see how much it's efficiency is, I'm going to assume that a purpose made TEG is made for these temperature gradients. This means, that taking a shower yields 80-160 Wh of electrical energy. I'm going to go with 100Wh. A family of 4 could create about 0.5kWh in a day, or 180kWh in a year. Thats about 40 EUR where I live.Now, lets take a detour, and investigate who pays for this. I am. The utility company measures the incoming and outgoing temperature and the used volume. There is energy conversation. But I only have to pay about 8.5 cent per kWh for hot heating and 22 for electricity. (There is a general consensus here, that district heating companies are pirates with a eye-patch. Due to obsolete laws, they are allowed to charge these high fees. I'm not going to go into this now). So, by the laws of physics and contracts, I have to pay about 15 EUR as heating bill for that 40 EUR worth of electricity.25EUR / year. It should be (ballpark) competetive with solar, so lets have a ROI of 10 years. This means that the heat exchanger, the Peltier element, and the inverter, including installation should be about 250 EUR. I should probably stop here, because that seems quite impossible. But where is the fun in that.

author: tszaboo
author_rank: Super Contributor
timestamp: April 26, 2020, 05:05:18 pm
content: So how much such a device would cost?To make an estimate, we need to know not only the energy, but the power. Lets say a shower is 10 minutes long, and generates 100W. So size the system for 600W.Inverter: Meanwell A301-600-F3 is probably a good example on how much BOM cost could be. It's price is 68 EUR @ 20 PCS. Exact voltages and currents have to be tweaked and maybe it needs an MPPT algorithm, that could increase the price of it. Also, I dont think this is meant to be run continuously, but it will not run continuously in the target application, so I give it a pass.Peltier module: TEC1-12705, seems to be plentiful and cheap. I am a bit puzzled by the sizing of the system, not 100% sure if it needs 6kW capacity of the elements or 600W. In the first case, we need about 200 EUR worth of modules, in the second case, only 20 EUR. I think it is the first. The surface is 400mm x 400mm. I used aliexpress pricing. If someone knows the best sales channels for these, let me know.Heat exchanger: Technically, this is needed in any case, but lets make a theoretical calculation. I think the TEG system is significantly larger. I probably dont have a good estimation for this, but lets make one anyway. 240x40 PC water cooling is sold at 12 EUR(Cpu Semiconductor Cooling Plate Radiator Water Cooling Plate Internal Flow Channel Extrusion Training 0.5 Mm Cooling 1 Pcs) Its for 6 TEG. You need 17 pieces for 400x400mm, thats 200 EUR. I think this is an upper limit for the price, in quantity, and optimized sizes probably half of this is doable.Piping, wiring, isolating, boxing. I add extra 50 EUR for this. Assembly is probably another 50, I'm just guessing here.Total 470 EUR. We ended up at twice of our target price with 10 years ROI.I dont take this as a failure. I think this might actually be worth it. I took a lot of liberty when calculating the price of some elements, especially the mechanical parts. If you have a larger family, the numbers become better.And I have a plan B, where the house's heating system is decoupled from the district heating. So the entire heating would go through a heat exchanger. That would yield a much larger system.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 26, 2020, 05:17:30 pm
content: I think the estimated TEC efficiency is too high. The usual thumb rule is 10% of Carnot efficiency for a good one. So with some 30 K of temperature drop (70 to 40 C - already need a nearly perfect heat exchanger for that) the Carnot efficiency would be about 10% and thus more like 1% max for the peltier element for heat to electricity.The nominal power for the peltier elements are at 0 K difference, so heat flow under favorable conditions. This would also be the heat that flow back internally at the max temperature difference. So it would be more like 2 x 6 kW of nominal power, as the temperature difference is only about half the maximum temperature difference.

author: tszaboo
author_rank: Super Contributor
timestamp: April 26, 2020, 11:50:39 pm
content: Quote from: Kleinstein on April 26, 2020, 05:17:30 pmMaybe it is.https://www.google.be/url?sa=t&rct=j&q=&esrc=s&source=web&cd=25&ved=2ahUKEwjL5L7xn4fpAhWJiqQKHW0bCcAQFjAYegQICRAB&url=http%3A%2F%2Fwww.asee.org%2Ffile_server%2Fpapers%2Fattachment%2Ffile%2F0002%2F3299%2FASEE_-_thermoelectric_paper_-_final.pdf&usg=AOvVaw2ZMnJiqo2xUBpUpYW3yLV5This paper measures efficiency with my selected TEG, and they measure about 1.5% at these temperatures. They also mention:QuoteThey also only generated 0.5W with this 60W nominal power Peltier element.

author: ejeffrey
author_rank: Super Contributor
timestamp: May 04, 2020, 05:45:05 pm
content: Yeah, if you pay 8.5c/kWhr for heat and 22c/kWhr for electricity you would need your TEC efficiency to be > 8.5/22 = 39%. That is far above the Carnot efficiency (~15%) for the stated temperatures and more than an order of magnitude above what you could get from a realistic TEC setup.The only way to theoretically make this work would be to use waste heat, either from the drain or to capture free energy from the entropy increasing process of mixing when the hot water is mixed down to shower temperature. In that case, it wouldn't cost any energy but the carnot efficiency would be even lower and the amount you would get out would be miniscule, so your chances of paying back any capital expense would be zero.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 04, 2020, 10:37:42 pm
content: At those prices, solar would be a more worthwhile investment.