Thoughts on a/c

dhilldhill Solar Expert Posts: 47
I'm going to need a/c this summer in the north Texas heat. Last year, I ran a 6000btu window a/c unit with my portable generator. It worked fine to run it, but this year I'd like to be able to conveniently cool the house without having to manually start my generator. I would like to plug in an 18,000btu window unit and let my 10k standby generator auto start as soon as the batteries pulled to 24 volts or so. The generator would cycle off/on as the a/c unit cycled off/on. Is it feasible to set up my a/c this way, or would I be unduly stressing my battery bank and/or inverter? I can always hook up the portable and manually start the generator, but I'd sure like to be hands free with a/c if I could run it that way. Any advice or comments are appreciated. Thanks.
Dusty

Comments

  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Thoughts on a/c

    dusty,
    i did move your post here as it is more appropriate here. do know that running an 18kbtu AC unit from solar is quite an undertaking and really isn't worth doing unless you're rich. i have one that big and it draws about 15amps at 220vac. that's 3.3kilowatts. heating and air conditioning are big power suckers and i recommend you trying other things like adding insulation or if it's dry there you can use a swamp cooler. another fellow from that state has stated that geothermal heat pumps are far more efficient. painting the outside of your house white too would reflect a great deal of heat away from you. if you were to run this for 12hrs of each 24hrs of the day you would need 39.6kwhrs per day from pvs. figuring 5hrs of full sun per day give or take yields 7.92kw in pvs minimumly. figuring losses this could jump to nearly 9 or 10kw in pvs needed. the battery bank too would have to be very large and larger if you wish a several day reserve. ouch, that's big. if you do this i want to see pics and a possible donation to my cause. :-D :roll:
  • BrockBrock Solar Expert Posts: 631 ✭✭✭✭
    Re: Thoughts on a/c

    I would be worried you would add a lot of cycling to your battery bank with the AC running through it. I do wonder if there is an easier way to start and stop the genset another way to only run when the AC was on, say somehow having the genset kick in whenever the loading on the batteries exceeds x number of amps?

    I should have asked though how large is your battery bank and how large is your array?
    3kw solar PV, 8 L16's, xw 5548, Honda eu2000i, iota DLS-54-13, Leaf EV, 4 ton horizontal geothermal, grid tied - Green Bay, WI
  • halfcrazyhalfcrazy Solar Expert Posts: 718 ✭✭✭
    Re: Thoughts on a/c

    we use a 6000 btu window unit as a dump load in the summer we just turn it on when the batteries are full. but one that large unless you have a large battery bank i would run straight off the genny you can simply set the generators auto start feature up to a ac thermostat then when house gets over a set point the genny starts and ac comes on when house is happy the thermostat shuts genny off witch in turn shuts down ac. another option would be to set the inverters generator auto start to activate on load amps so when ac comes on the genny starts.
  • System2System2 Posts: 6,290 admin
    Re: Thoughts on a/c
    niel wrote:
    i have one that big and it draws about 15amps at 220vac. that's 3.3kilowatts.


    Doing the math on that returns an EER of ~5.45, horribly inefficient by todays standards.  A common 10 EER of an 18,000 BTU/h AC would require 1.8 KW/h, a power requirement reduction of ~45%.  If it uses that much, replace it ASAP.

    Cheers,
    Bad Apple
  • System2System2 Posts: 6,290 admin
    Re: Thoughts on a/c

    One thing you might have to consider is a delay relay function, to leave out the load of starting the AC for a few moments until the genny gets up to speed and stabilized. Then connect the AC.
    Wayne
  • System2System2 Posts: 6,290 admin
    Re: Thoughts on a/c

    I think I am close to on topic with this, I read somewhere, where someone was running a slow pump from their waterwell through a car radiator and was using a 12 volt low CFM fan to blow air through it. I am on city water so I can't just dump it on the ground, would cost too much and city water is not very cold. I think well water is 65 deg? I am guessing you would not need much of a flow just the power to pump the water and run a fan. Maybe some insulation for the pipes.

    Just a thought.
  • System2System2 Posts: 6,290 admin
    Re: Thoughts on a/c

    LOL My uncle used that type of thing for years! Had a cold spring that produced lots of water, was able to syphon the water up to the house and back down to a pond lower than the spring - so no pump necessary, only a fan to drive the air through the truck rad.
    Worked fine for years, until he got too old to look after it.
    Wayne.
  • System2System2 Posts: 6,290 admin
    Re: Thoughts on a/c

    I am guessing by that statment it really will work.
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Thoughts on a/c

    i would think that would work, but why not tap it for the power and not just the temperature?
    bad apple,
    i could be wrong on those figures as i don't recall the eer being that bad. i am going on memory as i can't see the ratings stamp on the unit where i have it stored. if my memory serves me the other way here i think the eer was around 8.4. maybe the current was 13amps. i'm not that sure as it has been a long time since i bothered to look at those ratings and the unit itself is about 13-14yrs old. it far outdoes the one it replaced as that one was 11,100btus at 220vac and 15amps. it's so old it didn't have an eer on it. you thought i was going to do a joke huh? ok, you say how old is it? i say, it's so old they just started using freon the year before. don't say anything about the day job now. :roll:
  • System2System2 Posts: 6,290 admin
    Re: Thoughts on a/c
    jon wrote:
    I am guessing by that statment it really will work.
    Yepper, it will work, BUT how well it works will depend on how cold the water is and you should have lots of it to pass through the rad. If the water is already at room temp, guess what - - you just wasted your time and effort. But if you have a good supply of cold water (say 45 or 50 F, usually only available from a deep well or spring) it will work quite well.
    Wayne
  • dhilldhill Solar Expert Posts: 47
    Re: Thoughts on a/c

    Thanks for the responses, guys. I got lost in the move so I didn't see all of this til this morning. Thanks for moving the thread Niel, I should've paid more attention to the forum headings.
    I've been trying to figure out a convenient and efficient way to run a/c since last summer. My problem is I'm on the cutting line of humidity. Some days we are so dry a swamp cooler can work just fine, but many days, the air is too humid and the swamp cooler only makes the sweat sweatyer. My son works for an independent researcher/design specialist, and they are building effecient a/c units. The method they use allows the a/c to pull its largest load overnite so that it's running non-peak electricity while the costs are down. The design is creative, functional and interesting. Best of all, it works. But it still requires a great deal of electricity. I've been tellling him for months that if they could design a unit that could work with solar they'd get multi-rich. Of course, they believe they're going to do just fine with what they have, but it doesn't help me.
    The radiator system is good and works well. Others around here have used it. My well water is probably about 65-68 degrees, but in the hot summer sun, that water isn't going to stay cold long enough to run it through all the pipe it would take to get it to the window.
    I have pretty much decided that efficient isn't going to be part of the equation. Air conditioning is going to be expensive. I'll have to pay a price for staying comfortable. I suppose there're worse ways to spend money.
    My propane 10k generator can run an a/c unit without any problems. But I have that gen hooked to my inverter via the x-240. What I don't want to do is buy another auto/start generator because they are expensive. I suppose that is an option though. My original question really was whether or not I'd be stressing my solar equip (inverter, batteries) if I left it all hooked up as is and let the gen start up auto as soon as the a/c window unit pulled the battery bank to the cut in voltage. I considered raising the cut in voltage for the auto start to save on the battery bank. So, my options seem to be to a) continue to use a manual start gen, b) purchase another auto/start generator, or c) use the existing gen tied to the solar equip and let 'er run. I've realized that I'll can run a 12-15k btu unit. Anything higher is wired 220.
    By the way, I built as energy efficient as possible, lots of insulation, windows appropriately placed, shaded areas added, all of the passive that I could do given the design of the home.
    You guys who are genuises with this will get solar to Texas much sooner if you'll design an a/c unit that is efficient and convenient. We are spoiled by our a/c in the lone star state. 8-)
    Thanks for all of the responses.
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Thoughts on a/c

    dhill,
    even your son should know of geothermal systems. instead of bringing that water to the house, take the ac into the ground to be cooled by the water. arcandspark has stated that geothermal systems in place in texas are twice as efficient as a standard ac unit. this usually entails the laying of pipes in the soil near the surface to give or take heat. you can just send it straight to that water with less pipe and i can guarantee it'll do better than the surface pipe method. run it past him and see what he says. it still takes lots of electricity to run this, but higher efficiency does equate to less overall power used.
  • dhilldhill Solar Expert Posts: 47
    Re: Thoughts on a/c

    Thanks, Niel. I've met and talked to arcandspark. He's extremely knowledgeable and helpful. Haven't discussed geothermal with him. I'll have to send him an email and ask for his help.
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Thoughts on a/c

    good and tell him i said hi. while you're at it tell him to visit us here once in awhile.
  • System2System2 Posts: 6,290 admin
    Re: Thoughts on a/c

    Dusty, Long time no here from, hope you are doing well. Last time I saw you I told you about my friends house we were going to install Geothermal HVAC. It is far superior to any air to air AC units on the market. The highest SEER rating you see on most AC systems is around 14. The Geothermal systems are between SEER of 24 and 27. This is unreal. Rick Wedow is the local rep for the DFW area and we have talked about the system he sells. He even thinks the dual inverter system can handle the unit running on 220 volts, it draws between 8 and 12 amps according to Rick. I told him the problem would be it would be very hard on the batteries over a 24 hour period. Currently my friends system runs three 5000 BTU window units (5 amps @120 volts each) for two days before the batteries need charging. I would recommend you take a look at a web site called (Insulation 4 Less) and think about adding a radiant barrier to your house. I have seen their radiant barriers in action and I was very impressed. I am going to install their radiant barrier that they rate at R14.5. I dont beleive that a 3/8" thick mylar bubble wrap and give you R14 but if you install it with a 3/4" to 1" air space behind it, the performance is unmatch for the price you will spend. I have also included a web site http://www.f1-rocketboy.com/lister.html I am very interested in. It shows a very low 600 RPM 10,00 watt diesel generator that runs for over 12 hours on 3 gallons of fuel. That would be 10,000 watts of energy for 24 hours for the cost of 6 gallons of diesel. And in reality you really dont use 10,000 watts over a 24 hour period. This system would be perfect for an off grid battery inverter power source to recharge batteries. Far more cost effective than a reagualr propane, gas or even diesel generator we normally would look at. Also you could use the engine coolant to heat your domestic hot water and save even more energy. You can use waste oil from restaurants and turn it into Bio-diesel. There is a system for sale in Home Power Mag I saw on TV. They took 20 gallons of restaurant cooking oil and ran it through the machine and in about six hours ended up with 20 gallons of Bio-Diesel they put in a Dodge truck. The truck ran the 1/4 miile faster and had better performance then running on regular Diesel. The cost per gallon after running it through the machine ended up $1.70 per gallon which is much cheaper than diesel at todays prices. This slow running generator engine is based on a unit that USE TO BE MADE IN AMERICA, (what happened to America anyway) called a Lister Diesel. They were made back in the 1940's and many are still running today, used in Alaska for pumps. I am working on building a 24 volt 140 amp alternator powered by an electric start 18 hp propane which the inverter can start and use to charge the batteries with. Smaller, moe fuel effeicient and less work for the Trace inverters. They wont have to work as both DC/AC inverter and Battery Charger, thus last longer. Glad to here from you, I finally found a great job working maintenance at a one million sq ft church here in Dallas, the work never ends. Talk at you later, arcandspark
  • System2System2 Posts: 6,290 admin
    Re: Thoughts on a/c

    I did some research last year and found a great website http://mb-soft.com/solar/saving.html. Basically, this site talks about the benefits of using natural cooling found in the earth to cool your home. This site makes a very compelling argument for not using A/C at all, but simply putting multiple PVC pipes in the earth and using inexpensive fans to bring the cool air into the home. Our deep ground temperature (here in San Antonio, Texas) is around 74 degrees. It would be difficult to use ground cooling to help us, but almost anywhere north of us it should be great. I cannot fathom anyone not using this idea for cooling in the north. I really like this concept, as it is very simple. On the contrary, this same idea might be able to bring "warmer" air into the home for heating in the winter. Good stuff.
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