Oil in vacuum tubes

[Ampster]

I know the science.

Thermodynamics is an area of science that I have very little knowledge of. Care to share some of your knowledge in terms of some unit of measure of heat at a high level? What kind of volumes are you talking about and how much heat can you recover from the sun each day?

 

[WillyP]

My location is just south of the forty fifth parallel in other words the great white north. I think the trenches would probably have to be at least fifteen feet deep. The frostline is about six or seven feet. But yes I have looked into it. I currently live in Virginia (USA) where that method works pretty well. But I am moving to Maine next year. That is where I am building my house.

 

[ianganderton]

My location is just south of the forty fifth parallel in other words the great white north. I think the trenches would probably have to be at least fifteen feet deep. The frostline is about six or seven feet. But yes I have looked into it. I currently live in Virginia (USA) where that method works pretty well. But I am moving to Maine next year. That is where I am building my house.

My initial thought is that it would be more reliable in the winter than solar in that environment. Solar needs sun and heating needs lots of it

 

[Ampster]

My location is just south of the forty fifth parallel in other words the great white north.

I know there are some good sites to estimate solar PV production based on location. Are there good ways to estimate heat production based on location? I know with evacuated tubes temperature would not be a factor but like the solar sites insolation would be the biggest factor.

 

[ianganderton]

I know there are some good sites to estimate solar PV production based on location. Are there good ways to estimate heat production based on location? I know with evacuated tubes temperature would not be a factor but like the solar sites insolation would be the biggest factor.

Aren’t PV panels approx 20% efficient and solar water heaters about 50% efficient?

if so the insolation data would be pretty transferable as after all watts are watts

 

[Ampster]

Aren’t PV panels approx 20% efficient and solar water heaters about 50% efficient?

if so the insolation data would be pretty transferable as after all watts are watts

Yes, so PV Watts could be useful. @WillyP if you don't a good source for the possible heat recovery per day during the seasons, try https://pvwatts.nrel.gov/ .

 

[WillyP]

Heat loss is much more difficult to calculate that electrical energy loss because of influence of ambient temperatures and the ability to insulate the storage. H

This is the biggest problem. There are so many variables, it is almost impossible to simply plug in a mathematical formula and just calculate the needs. One of the biggest unknowns is; what is the normal discharge temperature of the water in a slab with pex pipes. Of course computing that requires knowing all sorts of other unknowns, like the slab temperature, the length of the pipes, the sub surface temperature. In my design I have tried to over compensate for everything. I figure this way if I am wrong, at least I will have more heat rather than less.
It also occurs to me that I have not posted the over all design to this forum. I'll do that in a minute.

 

[WillyP]

Anybody who has ever owned (or operated) a well-designed greenhouse*1 knows how well heat sinks work. However anybody who has owned a poorly designed greenhouse knows (without a doubt) that heat sinks are nothing more than the fantasy of wild eyed preppers, who believe heat loss is a government conspiracy. The truth is someplace in the middle.
I believe, the main key to building an efficient heat sink is: you must have an insulated barrier that goes well below the frost line. Without that barrier you will be trying to heat frozen ground. There also needs to be an insulated barrier that goes all the way to the wall system and integrates with the insulation system in the walls. A good rule of thumb is; start with a foot below the frost line, then add another foot for every three feet the frost line goes down. So if your frost line if two feet down, you should build a four foot deep barrier. If the frost line is six feet deep the barrier should be at least nine feet deep. This can be shortened by adding “insulation wings”. The wings are basically a panel of foam insulation laid flat below the frost line. I have learned this through working in several greenhouses.
SO I am building a house in central Maine. The frost line is about six to seven feet down.
The way I plan to do it is: I pour a six inch concrete wall to a depth of eight feet (approximately two feet below the frost line) with two foot wings on the bottom. In the top four feet I’ll install pex tubing, just like I do in the slab. But this system comes on in the fall. It serves to heat the soil around the perimeter of the slab. Without heating the house above it. Although there will be some minimal heating of the living space as well. By doing this we get a jump on creating a warm area below the slab. So instead of fighting the soil temperature below the slab, the soil is helping to heat the slab. Of course I’m installing solar powered systems so this method doesn’t cost a lot of money to operate.
The heater:
For my hot water heat. I am building a lean to green house on the south side of the house. It will be 10X16 inside will be a system that uses evacuated tube (vacuum tubes) solar collector, to heat oil. The oil will live in two fifty gallon tanks. We will call them ot1 and ot2. They will be connected near the top with an over flow tube. SO each tank will hold up to approximately forty gallons of oil. But the system will only have fifty gallons of oil in it.
Ot1 will have the intake for the solar heat collector. Ot2 will hold the output of the heat collector. When ot2 fills to the overflow tube, it will run off into ot1. Over the course of a normal day the oil should reach a temperature over 150°F. I believe maintaining a temperature over 140°F will be fairly easy.
From ot2 oil will be drawn to heat the water tanks. The oil from the water tanks will drain into ot1, where it can be reheated by the solar heater.
The water tanks:
I plan on burying two 850 gallon water tanks below the green house. In the green house (above ground) will be a 1500 hundred gallon discharge tank. Each tank will have a coil inside that hot oil flows through (from the above system). It should be noted the manufacture of the tanks recommends never draining them below 80% of capacity, so a 1500 hundred gallon discharge tank will be more than adequate.
I will label them wt1 wt 2 dt1
Wt1 will be one of the underground tanks, it will serve as the main feed for the radiant floor heater system. Wt2 will be the other underground tank and will serve as a slave for wt1. Basically wt2 will just hold hot water until wt1 needs it and then wt2 will pump into wt1. Any helpful ideas for making that work efficiently without a bunch of electronics would be greatly appreciated.
Dt1 will hold the water after it has gone through the radiant floor heater. Being above ground (in a greenhouse) and heated by coils of hot oil. It should easily reheat the water back up to 95°F then a heat sensitive valve will open and it will fill the below ground tanks (wt1 and wt2).*3

I am really hoping for some feedback from anybody who has built a similar system, or any part of it.
The main question nobody is willing to answer is. How much water does a typical radiant floor heater use, for a two thousand square foot house with excellent insulation.
Is 95°F hot enough for the input water of a radiant floor heater? I am getting all sorts of conflicting temperature recommendations. The most reliable information I get says 90-95°F *2is more than adequate.
Has anybody ever done something like this, with a barrier outside the foundation that goes below the frost line? All the science seems to work, but the internet screams never put in a slab without insulation underneath it. But I hear sometimes the internet can be wrong…
Thanks for reading this. I am looking forward to any helpful ideas, or discouraging information that stops me from doing something really stupid that costs a ton of money.

*1 By “green house” I am referring to a glass room, not a house with a “green rating”.
*2 90-95°F is a temperature I got off the internet. I am open to adjusting that up or down. That is one of the reasons I am posting here, and on other forums like this one. Any advice from people with experience, monitoring water temperature, in a concrete slab with pex pipes would be wonderful.
*3 I am using a separate discharge tank, so cooled water from the slab won’t mix with already heated water, until it has been reheated. The discharge tank will have a temperature sensitive drain on it. When the water is reheated to the proper temperature, it will refill the two underground tanks.

 

[Ampster]

Twenty years ago I put a radiant heating system into a house that I was remodeling in Southern California. It had a Southern facing series of double insulated Low E glass doors that were 9 feet tall and about 50 feet long. All of the floor area through out the house was two inches of a special form of lightweight concrete that had an additive that made it more thermally conductive for radiant floors. The house was on a raised foundation and I used a 1/4 inch reflective foil bubble wrap on the plywood floor before the radiant heating tubes were laid down. The Southern exposure of that part of the house did create a greenhouse of sorts that passively heated that part of the house. Because I was in Southern California near the ocean I rarely had to deal with temperatures below 40 degrees. The contractor installing the radiant tubing did the calculations for the amount of heat I would need and it was all done with a small highly efficient natural gas boiler that extracted so much heat that the exhaust was 4 inch PVC pipe. It was a zoned system that only needed to heat the bedrooms at night. In the morning the kitchen zone was warmed up. During most days the passive heat warmed the rest of the house unless it was cloudy and cold, in which case the boiler had to run.

As I mentioned this was in a temperate climate with a home that was already fairly well insulated and the contractor did all the thermal calculations. I chose radiant heat because my wife was asthmatic and radiant heat eliminated dust mites and all the air blown particles. I had also grown up in a house with radiant heat I had liked the comfortable heat it provided. The contractor told me that the 1/4 inch of insulation and the thermally conductive light weight concreted made some difference in improving the efficiency of the system but that was twenty years ago and I do not remember the details. Over the years as I have done other projects I have always been reminded that the most cost effective expenditures are always energy conservation expenditures.

The feed back that I would give @WillyP is to look at several design options of reducing the amount of concrete except where it is functioning as a thermal mass or is needed structurally. Insulate the exterior structural concrete from the interior thermal mass concrete. The interior slab can be poured over rigid foam insulation to separate it from the surrounding permafrost and that may be worth looking at from a cost perspective. I am sure you are already considering Insulating the structure for the greatest heat retention.

 

[Hedges]

I've thought about similar for in-floor heating of my house, but using a water loop, likely potable, in the manifold at end of evacuated tubes. (mild climate.) Currently I have a switch for gas/electric forced air and burn off my net-metering surplus with resistance heating. I would like a way to at least store heat from low-priced morning to high-priced late afternoon (3x the price)

Are you trying to have hot tanks and cold tanks, and level changes in them as you transfer? Pumps have to work harder. Although using full temperature water will heat the house faster, you would be leaving some energy on the table.

Phase change materials would be great, but I don't know any good ones to use for that temperature range and above. For cooling of course it's obvious.

Years ago the park department built solar-thermal hot water systems for rest stops. They were more economical than electric heat, less economical than gas heat.

Today, I can put in PV for 1/3 the cost of lowest tier electric rates. (Which in our location are 2x or 3x the rates people pay in some states.)
So that changes the equation for electric heating with PV, and heatpumps could too.

 

[Ampster]

..........How much water does a typical radiant floor heater use, for a two thousand square foot house with excellent insulation.
.............
90-95°F is a temperature I got off the internet. I am open to adjusting that up or down. That is one of the reasons I am posting here, and on other forums like this one. Any advice from people with experience, monitoring water temperature, in a concrete slab with pex pipes would be wonderful.

If you are talking about the water in the pex pipes that is easy. Calculate the spacing of the pipes and that will tell you how much pipe you will need. Using the inside diameter calculate the number of cubic inches of volume per foot. Multiply that by the total pipe length and divide that number by the number of cubic inches in a gallon and that is your number of gallons. Add a few gallons for a fluid reservoir and that is it.

If you are talking about the amount of hot water storage and oll storage then that is a more complicated question which would take a greater understanding of thermodynamics than I have. I know the concept is the hotter the water or oil is the more heat it contains. Part of the answer would be related to the exit temperature of the evacuated tube collectors. There are formulas out there that use some unit of measure that probably could be translated back to Watts. That probably starts with the calculation of how many BTUs of other measure of heat that it would take to heat a well insulated 2000 sq. ft. home in Maine for the winter season. Then you have to figure out how much heat your tubes could generate each month during different periods of solar insolation,

90-95 degrees F was the temperature that my system ran at so I think that is a good number for the water in the radiant system. The temperature of the oil or the water storage is another calculation. It is conceivable that you could store the water and oil at a much higher temperature and use a mixing valve to blend that with your water coming out of the system to run the radiant pipes at 95 degrees.

 

[WillyP]

If you are talking about the water in the pex pipes that is easy. Calculate the spacing of the pipes and that will tell you how much pipe you will need. Using the inside diameter calculate the number of cubic inches of volume per foot. Multiply that by the total pipe length and divide that number by the number of cubic inches in a gallon and that is your number of gallons. Add a few gallons for a fluid reservoir and that is it.

If you are talking about the amount of hot water storage and oll storage then that is a more complicated question which would take a greater understanding of thermodynamics than I have. I know the concept is the hotter the water or oil is the more heat it contains. Part of the answer would be related to the exit temperature of the evacuated tube collectors. There are formulas out there that use some unit of measure that probably could be translated back to Watts. That probably starts with the calculation of how many BTUs of other measure of heat that it would take to heat a well insulated 2000 sq. ft. home in Maine for the winter season. Then you have to figure out how much heat your tubes could generate each month during different periods of solar insolation,

90-95 degrees F was the temperature that my system ran at so I think that is a good number for the water in the radiant system. The temperature of the oil or the water storage is another calculation. It is conceivable that you could store the water and oil at a much higher temperature and use a mixing valve to blend that with your water coming out of the system to run the radiant pipes at 95 degrees.

I agree with all of that. My plan is to let the oil get as hot as it can, then run it through coils whenever the water tanks start to cool. I can do this using a simple thermostat connected to the pumps. the oil Pumps are cheap enough so I can use one for each water tank. This will keep the water at 95°F, therefore eliminating the need for mixing valves. Pretty much everybody agrees that reaching oil temperatures of 150-185 will be easy to do with solar tubes.

 

[Ampster]

So the big elephant in the room is how much heat will you need and how much can you generate? Are you giving any thought to a backup plan in case you have 10 days of cold cloudy weather?

 

[WillyP]

So the big elephant in the room is how much heat will you need and how much can you generate? Are you giving any thought to a backup plan in case you have 10 days of cold cloudy weather?

absolutely! I plan on having an on demand water heater that will heat water in the discharge tank.

 

[upnorthandpersonal]

The specific heat capacity of oil, and let's assume the maximum, is around 2 kJ/(kg K) . The specific heat capacity for water is around 4 kJ/(kg K), or twice that of oil. Source: https://www.engineeringtoolbox.com/specific-heat-fluids-d_151.html When using a well insulated tank, nothing beats water as a storage system for heat since you can store more energy per volume. In addition, water will also absorb heat faster because of its thermal conductivity.

A 1000L hot water tank at 80C stores 100kWh of energy (source: https://www.engineeringtoolbox.com/energy-storage-water-d_1463.html). You would need a tank twice the size to do the same with oil.

My place uses hydronic underfloor heating, i.e., hot water is pumped through pipes embedded in the concrete floor (which itself acts as a thermal battery). To heat the water, you have a thermal battery which is a 3000L water tank, which is coupled to a wood burner which can also take wood pellets if needed. Excess PV solar is also put into the tank, it's just hooked up to another heating element in the tank and switches to that when the battery is full. This is in a cold climate (63 degrees north) so hence the wood burner.

 

[svetz]

... nothing beats water as a storage system for heat ...

Phase change materials could. For heat storage in a tank, this article might interest you. For example, you could get 246 Kj/Kg at 36.7°C with Isocane.

 

[upnorthandpersonal]

Phase change materials could. For heat storage in a tank, this article might interest you. For example, you could get 246 Kj/Kg at 36.7°C with Isocane.

Yes, but most of those are not exactly practical at this time. You could use paraffin, but you have the disadvantage that it has a low thermal conductivity so getting energy into it is harder. For other reasons, molten salt would be excellent - but again, not exactly practical on the scale we're talking here...

 

[Zil]

In Maine drilled wells with recirculating water could be used for heating and cooling.

 

[WillyP]

The specific heat capacity of oil, and let's assume the maximum, is around 2 kJ/(kg K) . The specific heat capacity for water is around 4 kJ/(kg K), or twice that of oil. Source: https://www.engineeringtoolbox.com/specific-heat-fluids-d_151.html When using a well insulated tank, nothing beats water as a storage system for heat since you can store more energy per volume. In addition, water will also absorb heat faster because of its thermal conductivity.

A 1000L hot water tank at 80C stores 100kWh of energy (source: https://www.engineeringtoolbox.com/energy-storage-water-d_1463.html). You would need a tank twice the size to do the same with oil.

My place uses hydronic underfloor heating, i.e., hot water is pumped through pipes embedded in the concrete floor (which itself acts as a thermal battery). To heat the water, you have a thermal battery which is a 3000L water tank, which is coupled to a wood burner which can also take wood pellets if needed. Excess PV solar is also put into the tank, it's just hooked up to another heating element in the tank and switches to that when the battery is full. This is in a cold climate (63 degrees north) so hence the wood burner.

Thanks for replying. Is yours an open system? How do you keep the water in storage from over heating?

 

[upnorthandpersonal]

Thanks for replying. Is yours an open system? How do you keep the water in storage from over heating?

It's open - i.e., not pressurized. There is a temperature sensor probe I use to cut off adding any solar if needed. With the wood/pellet burner, you kinds know how much fuel you use based on the current temperature, so that is used to stop feeding x amount of fuel (or in the case of wood, I just don't put more).

And of course, the tank itself is it's own closed system, and the floor heating as well. That all just taps from the large tank with heat exchangers.