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Ask Jeff Yago
Solar & Energy-Related Issues

By Jeffrey Yago, P.E., CEM
Jeff Yago


Solar to radiant floor heat

I was wondering if it would be possible to use the old 'black hose under-glass' passive solar approach to under floor heating? I know this is a comfortable and energy efficient way to heat, and although one would need to have a way to turn it 'off ' during hot times, it seems an ideal application. Would one need a circulator or could the natural force of hot to cold work as a gravity flow system? What about condensation issues? This may be outside your realm as it does not deal with electricity, but it is 'solar' so I thought perhaps.

Thanks,

Cat Lyddon

Cat:

I strongly recommend underfloor heating to my off-grid clients as it requires very little electric power if you are using a gas or oil-fired hot water boiler. The water temperature flowing in the tubing does not need to be as hot as other heating systems since the floor slab absorbs all this heat which then slowly rises to heat the room. In most cases you will need a circulating pump, as the piping routing has many horizontal loops which makes it much harder to design a thermo-syphening system (no pump, operating only on heat rise, cool falls thermo properties of the loop).

Temperature control is not a problem as a wall thermostat turns off the pump or closes a zone valve when the room heats up, and starts the pump or opens the zone valve when the room gets cold. However, you cannot route the piping directly from the roof solar array to the floor slab heating loops for many reasons. First, the roof array loop will require anti-freeze in the water or will need to drain back or down when the sun goes down. Second, you will want hot water circulating in the floor when it is not circulating through the roof array - like at night! Also, some systems may allow air into the roof array when it drains back to prevent freezing, while you will not want any air entering the floor slab piping as it will cause the pump to quit pumping.

Most systems have one loop piped through the solar array back to a heat exchanger in a large insulated tank. This loop only pumps water through the roof array and back throught the heat exchanger to heat the tank water when the temperature in the array is hotter than the water in the tank. A second piping loop pumps this heated tank water through the special plastic high temperature piping buried in the concrete floor slab. There are also systems that have the tank water piped up through the solar array directly, with the heat exchanger piping connected to the floor slab piping loop. There are many design issues that determine which piping arrangement is best for a given application as there is no one right way to do this.

Finally, there are no condensate issues unless you are trying to cool the floor slab by running water through the solar array at night when it is colder, as condensate only forms on the piping when it is colder than the "dew point" of the inside air. If you get condensate forming on a cold glass of water, then the conditions are right to get condensation on the piping if it is as cold as the water in the glass. You do not form condensate during heating!

There are many contractors out there who do a great job of installing this type of floor slab heating. This requires very special plastic tubing that is also pressure tested before pouring the concrete as a leak later would be a nightmare to find and repair! In most cases they use a special lightweight concrete dsigned for this purpose, and the spacing of the loops and size of the tubing will change from room to room depending on heating load and water loop temperature. The lower the temperature of your solar heated water tank, the more tubing you will need to transfer this heat.

I suggest you talk to several of these contractors for ideas and assistance.

Good luck,

Jeff Yago


Propane vs Natural Gas

Can you supply information about using propane, and how propane is manufactured? Perhaps assume some small home, two people, and discuss what average usage and cost would be. Maybe discuss tank sizes and frequency of filling. How does the cost of natural gas and propane compare? For some of us folks who have lived with natural gas piped to the house, propane is a mystery.

Thanks

Bruce

Bruce,

If you already have natural gas piped to your house you would not want to consider propane, as this would add issues related to tank storage, filling, and a higher cost than using natural gas. However, propane is a wonderful fuel if you live off grid, as solar electric is way too costly to power kitchen stoves and water heaters which are perfect for propane. In addition, most off grid or rural homeowners who have a propane tank to fuel their kitchen stove and hot water heater can also supply propane to a backup generator. Any gasoline fueled engine can be modified to operate on propane by switching the carbarator and adding an automatic fuel shut-off valve.

When I was much younger we would drive by an oil refinery located along the highway in the next city. Day or night, 24 hours per day you would see multiple tall exhaust stacks with flames shooting high into the sky. They were burning off propane gas which is a "waste" product of making gasoline, and back then there were few restrictions on waste emissions. Other fuels were so cheap it was not worth the added cost of compressing this gas into cylinders and trucking it to a homeowner. If you had a backyard grill, you used charcoal, not propane!

When the price of natural gas and other fuels shot through the roof, propane became more profitable to supply to homeowners who were not near natural gas pipelines, and today's air quality restrictions has almost totally eliminated the burning off propane gases at these plants.

As to your question of tank size, it depends on the application. If you only have a propane gas stove and hot water heater, the supplier may install a stand-up type tank in the 60 gallon range. If you are also using propane for a hot water boiler or space heaters to heat your home, they may install a 500 gallon tank that lays on its side. This size tank will probably need to be re-filled 3 or 4 times during the winter and 1 time during the fall depending on your location.

Since they never fill any propane tank all the way full (it requires space above the liquid surface area to convert from liquid to gas), and you do not want to drain it completely, re-filling does not mean you are buying 500 gallons of propane per fill-up! Propane is a liquid and is "pumped" from liquid tank trucks into your tank using special hoses and quick-connect valves.

Sometimes you can request the tank to be buried underground, and only the top access cover and fill valves are above ground. If you are in an area that makes it hard for a truck to access during the winter months, you may want a 1000 gallon tank which will last longer between filling.

If you are considering installing a propane tank and natural gas is not available, be sure to locate the tank "down-hill" from your home or occupied area. Although these tanks and valving are extremely safe, propane is a very heavy gas and if there is a leak it will "flow" down hill just like water, hopefully into a creek bed or other wooded area where it can evaporate without ignition.

Any gas appliance designed to operate on natural gas can be converted for propane by changing out the fuel metering jets in each burner. Since propane is normally supplied at a higher pressure than natural gas, the small orifice in the jets need to be smaller. Any gas appliance dealer will provide these conversion jets at no added cost.

Hope this answers all your questions,

Jeff Yago


Solar energy to power air conditioner

I want to put a solar panel just for the air conditioner. We have a 60 hertz, 115 volts, 8000 btu AC but we don't know how many solar panels to use or what size to use. We have researched this topic and have come up with no answers and just more questions. Can you help decide what panels to use

Carolina Gonzalez

Carolina:

This is like calling up a car dealer and asking what car is best to haul 4 people and that is the only information you provide! What are you trying to cool, how many hours per day does it need to run, is it cycling on and off, does the compressor run full time, does it need to operate after the sun goes down or only when it is powered by the sun in the afternoon. As you see, there are many considerations, but that is really not important as I am betting you would never buy a solar system in the first place to do this.

Let me explain - going on only what you have provided - an 8000 BTU air conditioner. My energy data tables indicate an air conditioner this size will require about 900 watts to operate and consume 667 kWh per year. If this was powered from your utility grid, the 667 kWh translates into about $60 to $80 per summer using $0.09 per kWH which is probably higher than you are currently paying.

To provide 900 watts at 115 volt AC output, you would need a minimum of 1,300 watts of solar array for sunny days, and a much larger solar array for those days and weeks it is not as sunny or may be cloudy. This also assumes you will only operate the air conditioner during the 6 hours per day the solar system is collecting solar energy (from 9:00 AM to 3:00 PM max.)

The current average cost to install a solar power system is in the $8 to $12 per watt depending on if you have a battery bank or not (solar loads need to operate after the sun goes down). This means you can expect to pay somewhere in the $10,400 to $15,600 range for a system to power your air conditioner!

Like I said before, I do not think you would buy the system in the first place due to cost, and I do not know anybody in the United States who is installing solar power systems just to operate an air conditioner.

Solar power is the most expensive electricity you can buy, and it should be used to power only the most efficient appliances you can buy - fluorescent and LED lighting, high efficiency audio/video equipment and computers, slow turning ceiling fans - things like that.

So, the added $60 per year on your electric bill to run this AC unit doesn't seem so bad after-all!

Jeff Yago


Hot water on demand from an oil boiler

Jeff I have an older boiler I think its an HB brand - it gives me "endless hot-water" in the event the reserve tank gets used up the burner fires up and heats the water for the shower and also the tanks reserve. I saw a person take an hour long shower - ug.

It has a coil above the burner . With a coil I believe I somehow add something to this boiler to make it a hot water on demand heater. I think it would save a lot of oil if it did not have to keep this internal reserve hot . I have an automatic water filler attached and I believe when the water low it sets off a sensor which fills that inside tank and then the oil fired burner comes on and heats the stored water.... as it cools the burner warms it up again. I would rather it had no reserve and the burner came on when hot water was needed.

I think even in the winter if heat is called for then it can come on and start to heat the water rather than keeping some of it hot all along. It might take an extra few minutes to bring the water to boil but it might be worth it..

I have a hot water heater at the sink which boils the water for most things further saving on oil use.

I think most the needed items to have this work are already in the boiler. I am just not sure how it senses when hot water is needed and fires up the burner .. or how I would by pass the hot water tank.

Al Friedar

Al,

It may be possible to do what you are thinking about, but I do not recommend it. Here are a few issues that may make this not practical, but it depends on your boilers design:

1. You are trying to convert a hot water boiler into an on demand instant water heater. Many tank type boiler designs have two water flows, the flow of water around the burner then into the tank, and the flow of cold water into the tank bottom and the hot out of the top of the tank. The burner coil may not be able to take ground temperature water entering directly as they are designed to use "mixed" tank water which could be 90 to 120 degrees, not 40 to 50 degrees. Some coil materials can develop cracks and leaks when their very hot surfaces have a very cold water entering which produces a thermal "shock".

2. As explained in item #1, without the tank the water in and out of the coil would go into your piping system directly. Since the system was designed to use a tank, the very hot water (160 to 180 degrees) coming out of the burner coil normally returns near the top of the tank and mixes with the tank water resulting in a constant tank temperatures usually in the 140 degree range. If you pipe directly from the burner, the water out to your plumbing system will be what ever temperature it can be heated on a one time through basis. This means if you opened a hot water faucet for a very slow flow, the longer time passing through the heating coil could heat the water to a very un-safe high temperature, and a faster flow would result in lower hot water supply temperatures. At a minimum, you would need to add a tempering "mixing" valve to prevent over-heated water entering your system.

3. Your present system is controlled by a temperature sensor that turns on the burner when the tank temperature falls below setpoint. Instant type water heaters are controlled by a very sensitive "flow" switch, which senses when there is water flow starting through the boiler heating coil and fires the burner only during water flow. As soon as the flow stops, the flow switch shuts of the burner. If the burner does not shut off the instant the water flow stops, the water in the coil would very quickly turn into steam which will (hopefully) discharge out off a nearby relief valve before bursting the pipe. Even on commercially made instant hot water heaters, we have had problems adjusting this flow switch to get it to sense low flow rates through the coil, and many times the homeowner had to turn the hot water on full blast to get the burner to fire, then slow down the flow once the flow switch turned on the burner.

My advice is to insulate the storage tank as much as possible to reduce the standby heat losses, then when its time to replace the system, switch to an instant type water heater designed for this type service.

Good luck,

Jeff Yago


Cost to run floor fans

Jeff,

I am trying to figure how much it is costing me daily to leave on roughly 30 heavy duty floor fans that draw 6. amps.

Thank you,

Frank Vargas

Frank:

That depends on how you found out the amp draw was 6.0 amps. Most data printed on appliance nameplates is "worst case" electrical loads to obtain the UL safety rating, which is usually higher than the actual amp draw. However, to answer your question, I will assume this value is accurate.

The other item that will affect electrical usage calculations is your voltage. When we say a 120 volt wall outlet, it actually can be anywhere from 110, 115, 120, or 122 volts depending on transformers and voltage drops in your local power system. Again, for this question I will assume 120 volts.

ANSWER -

(30 fans) ( 6.0 amps) (120 volts) (8 hrs/day) = 172,800 w-hr = 172.8 kWh / day

(172.8 kWh/day ) ( 30 days/ month) ($0.09 / kWh) = $466.56 / month based on 8 hours per day and 30 days per month.

If we use 22 work days per month instead of 30 total days, this would be $342.14 / month.

For other hours per day, just change the values in the above formulas. Note - I am assuming you are paying an average of $0.09 / kWh for electricity. This is a good average, but some east-coast and west-coast states are as high as $0.14 / kWh and some mid-western states as low as $0.07.

Hope this helps.

Jeff Yago


Chinese solar panels

Dear Jeff:

I am curious, since Chinese solar panels cost about 1/5 the cost of American made solar panels, what is their downside? Are they poorly constructed or do they have poor connections or what?

Cordially,

Melvin Saunders

Melvin,

Ever hear about getting what you pay for? To be honest, I do not know any installations using them as they are just starting to offer modules to the US market. I do know several namebrand US inverter and solar charge controllers that have switched from manufacturing their electronics from their US plants to plants in China, and have been pleased with the quality they receive.

However, for years we purchased compact flourescent lights from companies like Oshram that have lasted over 10 years with no burn-out. Then we have purchased severel differnet compact flourescent lamps from different name brand American lighting companies that were actually made in China and these have been crap. Many lasting only a few months.

Building solar modules is an almost total automation process these days and requires very little labor. Most of the cost is in the raw materials, which cost about the same all over the world. Since most of China's low costs in the past have come from a lower cost of labor, I am not sure how this would provide major savings, especially since you then have the high cost of shipping large flat glass sheets without breakage.

I know of an American made solar module years ago that started failing at every installation after only a few years. The problem was traced to how they bent the tiny metal foil where it bends up from the bottom of one cell over the top of the next cell which is sealed under the glass. After a few years of heat and cold expansion, the foil would break at the sharp bends. My point is I do not want to install a bunch of these now, and have them start failing in 2 or 3 years, regardless of how cheap they are, and I do not want to be part of their manufacturing "learning curve".

Jeff Yago

P.S. American-made modules carry a 25 year warranty and I have solar modules powering my house that were made in 1980!


Compact fluorescent burnout

Hi Jeff,

I'm slowly trying to reduce my electricity usage and have been trying compact fluorescent bulbs in my standard sockets. The problem I've noticed is that wherever I have a fixture with multiple sockets the bulbs burn out in about 6 months. Where I have single socket fixtures I've got bulbs that have lasted 2 years now.

My 2 quick questions are:

1. Why does this happen?

2. Is there a type of fixture I can switch to that will alleviate this problem?

Thank you for your time.

Kevin

Kevin:

I am not sure how single verses multiple lamp fixtures affects lamp life unless the fixture has a cover and the multiple bulbs increases the heat around each bulb. However, if you read the email question I just answered yesterday about solar modules made in China, you will see I am not pleased with ANY compact flourescent lamps made in China.

When we completed our solar home in 1994, ALL of the lighting was flourescent or halogen. All room ceiling fixtures and corridor wall sconces were designed for complact flourescent lamps. These are the pin type base (PL-13) not the screw type base, but it is now 2006 and we have still not replaced a single compact flourescent lamp! These were all Oshram, which is now part of Sylvania. The Oshram lamps we purchased were made in Europe, not China. Now they are made in China.

Many "name brand" lighting companies have switched to China for their manufacturing, and it has been my experience that most of these lamps are crap. The same lamps from the same companies that lasted 10 years now last 6 months at best. I suspect your lighting problem is related to where the compact flourescent lamps were made and not the fixture they are in. I think all the old school lighting companies were used to making light bulbs that lasted 750 hours so they had regular replacement purchases. Then when compact flourescent lamps came out with 10,000 + hours life, they did not realize this would cut down on their re-order business. So these long-life lamps suddenly started to fail after only a few months. Hhmmmm..........

Hope this helps,

Jeff Yago


Battery discharge during cold times

Hi Jeff,

I used to have an Escargo roof top travel container outside on the ground which held 8 golf cart batteries. As you mentioned in your article, they needed to be in an insulated area where the temperatures never went below 50 degrees.

How can I keep these batteries outside of my small cottage without them draining energy in the winter months? I don't have enough room inside my cottage to keep them. Can I still use the Escargo roof top travel container to keep them in? The 8 golf cart batteries lasted only one year and started to fizzle out dramatically after the second.

Wendy

Wendy,

You already found out your car carrier will not provide proper protection for your batteries as it is not insulated, is not properly vented, and probably is made out of materials that do not like acid mist!

For small cabins with limited space we have used several different methods for battery storage. Since you are using golf cart batteries, there is always a danger of explosive gasses building up if not properly vented, so your battery enclosure needs to have a top access door that has a good seal and there needs to be a 2" PVC vent pipe near the top leading to the outside (with insect screen). Here are your options:

1. You could switch to sealed gel golf cart batteries which do not vent hydrogen gas and can be stored under a bed, under a counter or table, anywhere inside where they will be protected from the extreme temperatures. However, they cost 30 to 40% more than standard golf cart batteries and do not have any more amp-hour capacity. They also require tighter charging controls to prevent over-charging which will dry them out and they cannot be re-filled.

2. Some installers build an insulated battery box under the floor that has a sealed access door in the center of one of the rooms. Move a throw rug, pull up the access door, and the tops of the batteries are about 1 foot below the floor elevation. If you do this then don't forget they are there as they will build up more corrosion on the terminals due to the confined space and will still need watering. During equalize charging you will need to open up the door, open up the doors and windows in the house, and make sure there are no open flames while this is taking place.

3. You can build an insulated battery box against one side of the house (near inverter location on inside) with a sloped top that you can raise up to service batteries. You would use typical wood frame construction, only line inside with that concrete board they sell for tile backing around a bathtub. It comes in 2' x 4' sheets and is not cheap, so make your framing dimensions so you do not need to cut this board. Seal or caulk all interior joints. The floors, walls, and top lid need to be insulated and the exterior covered with something to keep out moisture. You still need a 2" PVC vent near the top.

4. Some people bury a waterproof plastic or metal box in the ground next to their house to hold the batteries, and place an insulated top over this like you would do for a well house. The box needs to be waterproof if your ground has a high water table. You can use one of those heavy metal or heavy plastic locked tool boxes made for pick-up truck beds. Some people bury an old refrigerator or top load freezer which are very well insulated and have a tight door seal, but don't forget, you still need a vent pipe or you may someday "blow" the door up into the sky!

Finally, you said you got 2 years of life from these batteries due to their lack of any protection from the heat and cold. You did not mention what kind of system you have, but if you are still connected to the grid and your inverter charging setpoints are not correct or you are not completing an equalize charge every 3 to 4 months, then part of this short life may be due to this. If you are solar only and your solar array is too small for your loads, the batteries may never be reaching "full" as you are using the battery charge as fast as it is going in. After several months of a low state of charge, these batteries will get a build up on the plates that will reduce their capacity just as if you removed some of the plates, and this is almost impossible to recover.

After you buy new batteries, I would check their charge level with a battery hydrometer later at night or early morning after they have been idle for a few hours and not being charged or discharged. Then check again several hours after being charged. Lack of a full recharge may have more to do with your short battery life then temperature.

Good luck and stay safe

Jeff Yago


Solar water pumping

Dear Jeff,

We want to collect water from a small creek and pump it up about 100 feet horizontally and 50 feet higher and store it in large container to use for watering a garden during the summer.

What size pump, solar array and container should we get?

What is the best way to collect water from the creek without disrupting its flow?

Please recommend some possible combinations of pumps, solar panels and storage containers for us to consider, including prices and sellers.

Thank you

Wendell Tangborn

Wendell:

Not enough information for a complete answer, but I can give you a few things to consider. We did one of these several years ago from a pipe in a creek that was washed out after a big storm. We went back and dug a very shallow depression in the rocky creek bank just beside the creek and dropped in 2 stacked concrete culverts on end, with the bottom setting in gravel. We then filled gravel in around the pipe leaving about 2 feet sticking up out of the rocky creek bank. We put a submerged 24 volt D.C. slow flow solar pump near the bottom which quickly fills with creek water.

We ran a 3/4" PVC pipe up the hill into the basement of the off-grid home where it fills a 500 gallon plastic tank. From this tank a second pump pressurizes the water, passing it through a sand filter (grit), a carbon filter (taste), and a final ultra-violet filter (kills bacteria).

Both D.C. pumps are powered by four deep cycle 6 volt batteries (24 volt) charged from a roof mounted solar 160 watt array (2 @ 80 watt).

We also designed a solar powered pump without batteries that pumped water from a lake, about 200 feet across a field, and about 50 feet uphill in elevation to fill a drinking tank for cattle. A float ball switch was in the tank to turn the pump on and off based on water level in tank. This pump was also 24 volt which works better than 12 volt for most pumping applications (less current required) and 2 solar modules at 40 watts each.

I have an article on this including photos and wiring diagram in one of the recent back issues. Go back about 1 or 2 years and look for "solar powered pump".

Good luck,

Jeff Yago


Battery wire

Jeff,

Thanks for such a great site.

I have been looking for info on how to build a small basic solar system for my off road pop up camper. Your Add solar to you truck camper is great. I do have a few questions.

I want to store my deep cycle battery in the camper. What gage wire should I use to run from the alternator to the battery in the trailor. I would like to remove my solar panal to move it around to keep it in the sun. Maybe 20 to 30 feet. What gage wire will that take. Do you know a web site that might sale 12 volt stuff?

Thanks,

Dennis Philen
Aspen, Colorado

Dennis:

For the amount of solar panel, voltage, and length of wire you would have for a moving solar panel system, I would use at least two #10 USE-2 single conductor copper cable (+ & -). If you go longer than 30 feet I would increase to #8 wire. The "USE" type single conductor cable is rated for outside, direct buried, in rain, in sunlight, whatever! The (-2) means it is rated for very high temperature like you would see inside the junction box on the back of a solar module in the sun.

The cable from the truck to the battery depends on the length and if this is going to be used for just charging only, or if it will carry any DC loads. For charging only, your alternator will probably be able to charge up to 60 amps, so in this range you can get by with #6 copper wire. However, if this cable will carry any large loads like an AC inverter, then you may need to go up to a #2 gauge. However, regardless of what wire size you use, be sure you have a DC rated fuse of the proper amp rating for the given wire size.

Watch out trying to use residential type wire from Lowes or Home Depot for this wiring between truck and camper batteries as house wire is very stiff and can break from constant vibrations and movement in mobile applications. Boating and marine stores stock these larger wire sizes in red/black covering that is water and sunlight proof, and has many more strands of copper so it is very flexible.

Please re-read the camper article where I address using a battery isolator. If you hook up a totally discharged RV battery directly to your truck battery, you could have so much current flowing that you can melt even a large wire. This could happen if you run down the camper battery during the night then try to hook it up to the truck the next morning. If you left the camper battery connected to the truck to avoid these high currents, you could also run down the truck's battery then not be able to start the truck. The battery isolator allows your truck's alternator to charge two separate batteries while keeping them and the loads connected to each totally isolated from each other. You can find these devices at car/truck parts outlets like PepBoys, but there are several different types to match different alternators so be sure to get the right model They cost around $25.00.

You can find all kinds of 12 volt DC lights and appliances at Camping World (www.campingworld.com) and any boating/marine center.

Glad you liked the article.

Jeff Yago



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