Greedy Goblin

Friday, February 10, 2012

LFM Engineer in heat pump field

I apologize to most of my readers as this post is not about WoW, games in general, business or even the M&S. It's about an engineering idea what I want to discuss but have no idea how to reach proper people on this field. However I try to write it in a way that everyone can understand my point so anyone who bothers to read it can understand.

Heat pumps are a very effective tools to heat a building. Their theoretical efficiency is around 600-1000%, and even cheap units that you can buy for your home are capable to 4-500%. What? Perpeetum mobile? Of course not, this efficiency (or more accurately Coefficient of Performance) means heat/electrical_energy. The heat pump doesn't create heat, it pumps from one place to another. When the heat moves from outdoors to indoors, we talk about heat pump heating, while the other way is called air conditioning. Its basic cycle is to compress gas, so it condenses while releasing heat (in the heated building), then lead the liquid out and de-pressurize while taking up heat (outside).

That's great, cheap and clean heating for our homes! Well, it is great and used in many places, yet it has limited use. Its problem is freezing. While theoretically you could pump heat while the outside temperature is below freezing point, actually you can't because the humidity of the air freezes on the evaporator unit, disabling it. To operate a heat pump you need a heat source that is above freezing point and has enough heat inside that it won't be frozen by the pump. So heat pump heating is more commmon in hotter zones where the winter temperature is usually not freezing (like Florida or Spain) or there is huge non-freezing water body is nearby (a lake, river, ocean). Alternatively you can build huge network of pipes or deep wells underground to suck out the geothermic heat. This has huge installation costs while the previous usages are situational: you either live in Florida or you don't.

However I found three heat sources that are never freezing and available to every household or commercial building and most industrial buildings too. If I'm right (and verifying it needs the expert) huge amount of fossil fuel can be saved, about 7-10% of the fuel used for heating buildings. That doesn't sound much but I'd like to remind you that every % decrease in heating use of fossil fuels decrease the carbon dioxide emission by about one hundred million tonnes per year. So if the idea works, it is huge.

The first heat source is waste air heat: you must ventilate the buildings to let oxigen in, carbon dioxide, humidity and bad odor out. When you do so, you lose the heat difference between the leaving warm and the entering cold air. Between 22oC and 2oC (which is the practical limit of a heat pump), 1m3 air contains 26kJ heat. But there is more in there as the air becomes humid from the various water sources in the heated room, like plants, breathing people, wet clothes drying and wet bathrooms. Counting with 50% relative humidity at room temperature, 1m3 air has 10g water steam inside. Steam has lot of energy, that little 10g water releases 23kJ heat if condensed. So if we don't vent the air just out of the window but drive it to the evaporator unit of a heat pump, you can recover almost 50kJ heat from every m3 air. If you replace the air fully just once in a day in an average home, you let 50*300 = 15 MJ energy go waste a day. That's about half kg of carbon dioxide.

The second heat source is heated waste water. Showering, washing, dishwashing all done by warm water that is just leaves the building into the sewers. If it would go to the evaporator unit of a heat pump, it could be recovered. An average household uses 100 liters warm water a day. That hold 100*30*4.2 = 12.6 MJ energy.

But here comes the crown jewel: if you burn fossil fuel (gas, coal, oil, wood), not all heat of the burning is gained by the building. About 15% of the energy leaves in the chimney as hot exhaust gas. There are already technologies to recover some of this energy, modern gas heaters cool down this gas to 40-50oC, but that gas is still hot and contains lot of steam. Leading this exhaust to the evaporator of the heat pump would recover this heat. How much it is? Equal to 15% of the fossil fuel burned in a simple heater and 6% on the most advanced one. That's a lot of fuel.

Of course, like any random idea of mine are free to use. So if you find this idea useful and would like to implement somewhere, feel free to. Just watch out for one thing: maybe someone else figured this out and patented it somewhere. If you know existing usages or has a question, the comment section is here for a reason!


Anonymous said...

Was thinking of getting a heat pump fitted rather than the current boiler i currently have (gas prices are taking the piss...) However i heard of a few stories where people had, had them fitted and their electricity bills shot through the roof due to the pumps that need to be driven? So this is something that may need to be checked out?

Gevlon said...

The heat pumps run on electricity. While they are much more efficient than heating, the electricity is more expensive than gas (as electricity is often made in gas plants running on 40-45% efficiency).

Anonymous said...

Some of your ideas are already incorporated in building houses adhering the Minergie standard here in Switzerland:

Especially ground-probe heat pumps are interesting if you can afford the initial cost (you get a little subsidy but it's still expensive) since they are basically available everywhere.

Water heat pumps are used in more large scale projects when you have a large water body nearby. This is due to the fact that if the water body is large and deep enough at the bottom the temperature is always 4C since it's the temperature in which the water is most dense. This allows to tap a relatively high temperature heat source even in very cold climate. My old school's complex was entirely heated by one of the first water pumps extracting heat from the bottom of the nearby lake.

Rodos said...

Your third point sounds a bit like a Trigeneration system. Burn fuel to generate electricity and use the waste heat for heating or cooling a building.

There's a significant push towards this type of system for new buildings in my city (Sydney, Australia) at the moment.

Anonymous said...

Few systems here that extract heat from the exhaust gases of boilers, by using heat exchangers. No doubt the efficiency could be improved upon though?

Anonymous said...

Have a look at the Icestick which actually thrive in temperatures below zero:
I havent tested it but it looks interesting.

Anonymous said...

Well, the problem with the exhaust gas proposal is that you need hot exhaust gas for the Stack effect. Theoretical, you can cool the gas down to roomtemperature and get near 100 % from the energy from the fuel. But then there is the chance that you poison yourself with Carbonmonoxid.
See for some basic information.

Josh said...

The idea is a good one, incorporating these 'waste heat' sources into a heat pump would definitely increase its efficiency. (Of course you would still need some source of heat, otherwise you would be trying to use heat created by the pump to run the pump, which violates laws of thermodynamics)

I would question its practicality, that is, in order for the exhaust air from a home to be used, most of the rest of the building would have to be completely sealed to prevent losing heat in those places. Similarly, waste hot water would have to be routed to the heat pump before it is put into the sewer system. These are less of an issue if the house is built around the idea (as previous comments' examples have shown).

Goodmongo said...

You tie into your post CO2. Well if you really do believe in AGW then keep up the use of fossil fuels and your heat problem goes away.

But on a serious note (since AGW is not serious) the cost benefit ratio needs to be factored into the equation. In the northern areas of the US there are a number of homes that use the exact same technologies that you describe. The problem is that while they are much more fuel efficient they are not economical. The absolute best payback is around 18 years and that uses some very favorable replacement and maintenance numbers which are probably not realistic.

A number of cost benefit anaylsis studies even show no breakeven point. The only way these technologies end up beneficial is if fossil fuels increase in price to around $180-$200 a barrel for oil and $9 for gas. In the US natural gas is being discovered at an ever increasing rate and the price is down to around $2 per 1000 cubic feet.

So for the next 100 to 200 years natural gas will be very abundant and pretty cheap.

Anonymous said...

>(since AGW is not serious)

I hope no-one is going to take this troll seriously and derail the comment section.

On-topic, though: I think some of those ideas are in semi-common use. Some cities (?) recover heat from the waste water in sewers. I also believe some ventilation systems pump the heat from air before expelling it, but this I am not so sure of.

Hombre said...

I'm having a hard time imagining the real world scenario that would be solved by this.

Large-scale rollout of heat pumps as I know it takes place with underground reservoirs (and those places are generally all-electric so the third solution wouldn't apply).

So you're talking about small-scale rollout, I suppose: an existing house being refurbished maybe? Refurbishing with a heat pump is rather costly and so is engineering your house to lead waste water and heat to your reservoir.

Can't be a money issue then, but if money is no problem and you're driven by CO2-reduction, why not install solar panels on your roof? Probably has a bigger yield, both financially and CO2-wise.

Maybe you have a specific scenario in mind?

Gevlon said...

I'm tired of deleting idiots telling "the second law of thermodinamics don't allow it". READ THE DAMN linked article about the heat pumps.

I did not invent heat pumps. They WORK, you can find one in your home, the refrigerator.

All I suggest is to use various wastes as heat reservoirs.

Anonymous said...

@Goodmongo, "heat issues" go both ways. You might need to heat the house but also need to remove heat from it. Global warming's theory is about a few degrees of higher global mean temperature but which result in much more varied temperatures during the different seasons, so actually the problem becomes worse as you have to heat more during the winter and cool more during the summer.

About break-even: a house is a very long-term investment which might very well get enjoyed for generations. 18 years is not a lot of time if you consider a house might last a century. The energy saving will be enjoyed as long as the house stands and is well mantained.

About natural gas, when Gazprom announced they were going to reduce the amount of gas they provide to Europe some countries had to tap into reserves. If for some reason this source of gas is reduced further or goes away many European countries would be in a very bad situation. Reducing your dependency on something you actually have no control over is a very interesting long-term investment.

Wilson said...

Gevlon, you're a fool. You said you wanted to talk to an expert, I gave you my expertise, and you deleted it. I don't need to read a wikipedia article on heat pumps, I can explain the theory behind them just fine. It's what I do for a living.

Heat pumps work because they use fluids such as freon with certain phase change characteristics. Air with water vapor does not. You put it into your heat pump, and nothing will happen. No heating, no cooling, no condensing, no evaporating. Just air going round and round in a loop. This is why the Second Law matters.

Claiming you've got a solution to a heat transfer problem without understanding the basics of thermodynamics is as idiotic as claiming you've found a better way to get to Mars without understanding the basics of gravity.

Goodmongo said...

Very few people stay int he same house for 18 years. You can't determine the BE point based on the house but on the person that paid for it.

Secondly, that is the most optimistic BE point produced so far. many runs show that they never recoup the initial costs. And if interest rates go up the BE point is pushed even further back.

I'm not up on natural gas supplies in Europe but have followed them here in the US. The proved reserves (wet) in the US have gone up from about 160 trillion cubic feet in the mid 90's to over 280 trillion cubic feet today. Meaning that much more gas has been found than used during this time period.

Bottom line is the technology works. But it is not economical to do.

@Any at 17:43 not a troll. Just not gullible (or subscribed to that religion) about AGW which is not proved.

Gevlon said...

Wilson, you are annoying and dumb and clearly not working on that field.

I never said I want to put wastewater or exhaust gases INTO the compression-evaporation cycle.

I said I want to use them as heat source.

Let me simplify it for you: imagine an air conditioner (a heat pump) which has the outdoor unit in the chimney so it can operate whenever the fireplace is used. Since the chimney is warm, the outdoor unit will not freeze unlike when it tries to operate on ambient air.

tweell said...

The ROI time is useful when paired with the life of a heat pump. An average heat pump used to heat and cool a house will last 15-20 years. Therefore the investment gets paid off at about the same time that the heat pump needs to be replaced. Since the heat pump is going to be the majority of the investment, you're barely breaking even at best.
Now if you're doing this anticipating that resources will be available but much more expensive than they are now, then the ROI would need to be calculated with that in mind. Alas, my crystal ball is at the repair shop, can't help you there.

Ingmar said...

The apartment I'm renting comes with a NILAN VP18 ( for specs), which ventilates the place while recovering heat from intakes in the kitchen and bathroom and uses it to both heat outside air (vents in bedroom and living room, thereby providing ventilation to the entire apartment) and also to heat up water in a tank that is used for hot tap water or simply to store it for later air heating (with a supplemental electric water heater so you don't get cold showers when you weren't generating enough waste heat). It also uses a heat exchanger to pre-heat outdoor air with indoor air, which makes it more energy efficient than a pure heat pump. The only thing it doesn't do is recover energy from waste water (probably due to the additional plumbing required and issues with dirty water clogging up things quickly).

All this works reasonably well in english weather and has a defrost function to deal with the heat pump icing over. However even in a new, relatively well insulated apartment it doesn't quite manage to heat things up sufficiently on cold days (freezing outside), which is why two rooms have additional electric heaters, but that's only needed a few days a year around these parts. It's also got some issues that are mainly down to lousy firmware coding in the controller. Oh, and due to the hot water tank being used for heat storage, tap water temperature can vary widely (between 40 and 65ÂșC based on settings), so you need a thermostatic shower mixer if you don't want to constantly adjust the temperature.

Anonymous said...

The limitation of heat pumps in cold climates is not icing; that's merely a nuisance and easily corrected. Instead, it's that the efficiency falls off in cold weather to the point where you're better off with a conventional heating system. Your proposals to nothing to address this issue.

Anonymous said...

@Goodmongo, depends on culture. In some countries it's usual to families to build a house and live there for generations. Still, even if you decide to leave the house either you will rent it or sell it. In both cases a low energy house has a higher market value.

Doing the BE in terms of economical savings for the owner is the obvious way to evaluate the investment and it can be easily computed, but it does not take into account returns due to less pollution, less need to build powerplants or buy energy from external sources, less reliance on foreign fossil fuel ecc... These returns become evident only after a critical mass is reached.

craig said...

This is a very common practice in industrial factories, etc.

Anonymous said...

@tweell, most of the investment cost in a house heat pump comes from the ground probe which will last far longer than 20 years. It's actually usually built with redundancy to avoid ever having to re-drill it which would be very expensive with the building in place.

Anonymous said...

A french startup proposing a system based on the second statement, using used and heated water.

Paul said...

There are already condensing furnaces that extract the latent heat of the water of combustion. Condensing this water causes problems with corrosion, which drives up the cost of the furnace. Some of these units do not have a chimney, since there is insufficient lift. Instead, the exhaust is ducted out the side of the building with a fan.

As for air exchange: the proper solution there is an air-to-air heat exchanger, which transfer heat from the outgoing air back to the incoming air.

Anonymous said...

Gev, the science is absolutely sound, no problems there. You named three convective cooling methods, where heat is carried away in a medium. There's also conductive (like having slab-on-grade foundations which lose heat directly to the ground) and radiative cooling - windows are the primary radiative heat loss in a building. Up to the theoretical thermodynamic limit, the heat lost can be captured and returned via heat pump, sure.

The problem isn't the science, it's the economics. The incremental cost per joule of developing some kind of waste heat capture and return system is massive compared to the cost of simply making your building more efficient at keeping heat from escaping in the first place.

In almost all cases, you can do way better for your money by increasing the insulation value of the building envelope and lowering the Air-Change-per-Hour rate by properly sealing windows and doors. For large buildings, they usually do recapture a significant portion of the heat from exhaust air via return air and air-reheat systems.

In the greater scheme of things, one of the best things you can do with waste from combustion, air heating systems and waste water (not including human feces) is to put them through a greenhouse first. You don't need a heat pump, just a big set of plants that cycle through building waste and produce something you can use later - food and oxygen! Buildings whose outer envelope is a greenhouse are more efficient, better looking, and provide a tangible return on investment in a way that active heat pumps can not.

Anonymous said...

Do you think you could gather more heat from used air than could do?