Off-Grid Chiller Unit?

[HotFlash]

Hi all,

I apologize ahead of time for not doing a completely exhaustive search, but I'm becoming somewhat exhausted searching, as it were. Maybe one of you could give me a jump-start in the right direction?

What we will have in a house we are modifying is a big tank of water as a thermal mass, used mostly for heating in the winter. Heat distribution will be through a 10-zone, forced-air hydronic system. The same hydronic system will be used for cooling during the summer. Our plans were to accumulate surplus PV electric throughout the year to apply to our summertime cooling -- running a heat pump to chill the hydronic working fluid.

What we would like to do is to minimize our reliance on grid-tied electric for various reasons having to do with questionable practices and policies of our local utility.

My thought is that we could use off-grid PV power to drive an inverter to power a small mechanical chiller (modified A/C) to chill our hydronic water during the daytime. We could use a small battery storage to buffer the system for short durations (e.g. passing cloud, bird, squirrel, whatever) and have the system shut down when the battery drains off to a certain level, only to be restarted after the charge has been built back up. The idea would be to dump as much heat as possible, as fast as possible, whenever the sun shines -- with no "real" reliance on battery storage, beyond buffering insolance fluctuations. Our "real" storage would be in the form of "coolth" in our thermal mass tank.

Has anyone done this -- or anything like it?

[Naptown]

Just put in a PV system that will generate all year and more in the summer. Virginia has net metering laws. Now will need to keep the system under I believe 20KW to avoid the stand by charge that Dominion has instituted. (Well they do own the state house there)
Batteries will eat you alive in installation and maintenence costs. If you can wrangle a time of use rate you may be able to run the chiller during non peak periods and draw from the storage during peak times.

[HotFlash]

Well, yes, it's the standby charge that first gave us a clue that we would be subject to the winds and tides of Dominion and the best legislature that [their] money can buy. It seems there may be a bit of a moving target in the solar rooftop game. Our system will be small and our demands low, but is it possible that standby charges could someday be imposed on even smaller systems (now above 10 kW, but later maybe 5 -- or even on all systems)? Perhaps as more solar rooftops come online, the justification could be pitched in the legislature?

And then there are some rather oppressive policies with regard to how net metering is done/charged. If you begin your yearly cycle on an energy deficit month, you will be paying up for power in the beginning. Then towards the end of the year you may end up with a surplus that will be reset to zero at the end of the year (I think they pay you half). Then continuing into the next year, you buy your surplus back at full price in your deficit months. I think it would pay to come online at the beginning of your surplus months to make the net metering pay off well. But when and how will the rules change?

Basically what we're trying to assess is whether our rooftop PV could be taken off-grid to meet the bulk of our summertime needs, thus avoiding the odious standby charge, or whatever else Dominion and their legislature throw at us.

[Naptown]

Well, yes, it's the standby charge that first gave us a clue that we would be subject to the winds and tides of Dominion and the best legislature that [their] money can buy. It seems there may be a bit of a moving target in the solar rooftop game. Our system will be small and our demands low, but is it possible that standby charges could someday be imposed on even smaller systems (now above 10 kW, but later maybe 5 -- or even on all systems)? Perhaps as more solar rooftops come online, the justification could be pitched in the legislature?

And then there are some rather oppressive policies with regard to how net metering is done/charged. If you begin your yearly cycle on an energy deficit month, you will be paying up for power in the beginning. Then towards the end of the year you may end up with a surplus that will be reset to zero at the end of the year (I think they pay you half). Then continuing into the next year, you buy your surplus back at full price in your deficit months. I think it would pay to come online at the beginning of your surplus months to make the net metering pay off well. But when and how will the rules change?

Basically what we're trying to assess is whether our rooftop PV could be taken off-grid to meet the bulk of our summertime needs, thus avoiding the odious standby charge, or whatever else Dominion and their legislature throw at us.

It could but the cost would be on the magnitude of 8-10 times what you would pay dominion every 5 years.
You are looking at an additional cost of 8-10K for batteries and replacement every 5-7 years if you buy top of the line batteries at a cost of 6-7K and they may only run your chiller for an hour or two, and as soon as you introduce batteries you will go from 80-85% harvest to maybe 60%
Build the house, live in it to get an idea of annual KWH use and then add the PV so you are not exceeding your annual use.
I am also not a huge fan of space heating in your climate. It is a huge investment that will sit mostly idle for 2/3 or more of the year.
If you insist on spending money install a geothermal heating cooling system, Grid tie PV to offset that without batteries.
That will reduce your carbon footprint and give you the biggest energy savings for the $ invested.

[HotFlash]

Thanks, Rich! I agree about assessing the kWh usage/yr before mounting up the panels, but we're needing to plan the rooftop, sun angles, etc. before we build. That forces us into making a first order approximation (a.k.a. wild-a$$ed, semi-educated guess) as to our solar needs beforehand. We also have to approximate the needed size of our thermal mass, etc.

I have to disagree about zoned HVAC. That's how we currently live, and although it might not be everyone's cup of tea, we've cut our energy usage practically in half using the (somewhat ineffective) zoning we have in our current house. Most people only live in half of their house anyway. We're no different. We're also not "crank the thermostats" sort of people. Most people laugh at how we use energy. We generally run about 66F in the winter and 80 in the summer, so our heating/cooling demands are pretty modest. We've gone mostly to CFL lighting and are considering a switch to LED. (That's hard for me, because I'm a photographer and care very much about the spectral quality of my light.) Our electronics are rather efficient, and we turn everything off that we aren't using. We dry our clothes using solar energy (clothes lines). We're pretty frugal people, really.

BTW, I'm not talking about much reserve capacity in the battery backup. If I can buffer the electric for a few min, I'll be very happy. I just don't want the power to the compressor to drop out if a squirrel runs across the panel or an airplane flies overhead. I'm thinking I could design some sort of controller that would shut the compressor down after a few min dropout of sunshine and then only restart after a minimum delay if sunshine reappears. I'm thinking in terms of a few micro systems running in parallel -- cannibalized window A/C units, running at 500 or 1000W -- rather than a 5 ton monster of a compressor. (We currently have 4.5 tons total, but according to my calculations we can probably run on about 2.5, especially if it's for A/C only.) This will be important for other reasons, too. We lose electric quite frequently and need to keep our systems small enough to run off of our generator (assuming no solar). Currently the HVAC is not wired to the generator because it is too large.

[Naptown]

Then consider several multi zone mini split units this would give you the zoning you desire. Now make sure you insulate interior walls between zones as best you can or zoning efficiency will be cut dramatically.
Plan roof space for at least a 10 KW system and leave enough space for a domestic hot water system.
You really are wasting your money on cannabalized window units and trying to convert them to chillers the SEER is very low on those and the work involved would be difficult at best. This is not to mention that as soon as you modify something like that you will void the UL listing.

[Sunking]

Hotflash you need to get up to speed fast, because half of what you want to do cannot be done.

You have three options:

Grid Tied
Off-Grid Stand Alone
Hybrid

Grid tied is the best option. The other two are extremely expensive and you will never obtain either an EROI or ROI. It is physically impossible, especially with your dirt cheap electric rates even brings GTI into question for ROI

[HotFlash]

cannot be done

I'm afraid I've heard this countless times throughout my half century of existence, perhaps because I've never fit in any boxes. I've found it not to be true at least 95% of the time. Heck, my entire dissertation was based on a theoretical finding of mine that an entire room of acoustical engineers (essentially the entire department) told me was impossible, misguided, and even "silly." I left that room having taught them all something new in their own field, and one of the profs in that room eventually joined my committee. Not bad for a neurobiologist.

So I'm wondering why rooftop heat collection (hot water) cannot be used to heat a home. Or is it hydronic heating and cooling that cannot be done? The home we live in now is hydronic, albeit heat only. My dorm was both heated and cooled with a campuswide hydronic system back in the late 70's. My lab was hydronically heated and cooled via another system on a different campus. The chiller plant was in the next building. Or is it the passive solar heating? I have a coldframe that reached 115F in the dead of winter before I installed an automatic vent -- single pane glass, R4 insulation, and rather large cracks around the lid. Now I can grow lettuce throughout the winter in a roughly 80F environment. Even our glass porch with leaky single-paned jalousie windows all around it manages to get warm on a cold winter's morning. I don't mean to be a smart-a$$, but it's not as though I haven't spent a bit of time researching these ideas.

FAIW, grid-tied PV is part of our plan. However, I continue to evaluate options -- even hair-brained ones. It doesn't hurt to dream a little and to think about these things, does it? Why be conventional? Before investing in a bunch of silicon, and even before building the roof system where that silicon will be mounted, I'd like to weigh both a Plan A (grid tied) and a Plan B (off grid). That's all.

I admit I have more to learn. That's why I'm here.

[HotFlash]

Rich, those multizone systems are nice and definitely worth thinking about.

According to my preliminary spreadsheets, the insulation between zones won't be necessary, as the active zones will "share" a small amount of their climate control with the inactive zones. I don't want the inactive zones going too warm or cold. During the winter I'll still want to heat the inactive zones to about 50F.

As for the SEER of the window units, you're quite right. However, the inefficiency is mostly a matter of the fan and coils. My heat exchange will be fluid to fluid, omitting the coils. We live on the water and will be shedding our waste heat through a closed loop system running to the end of our dock. I hadn't considered the UL issue. I think you're suggesting there could be insurance issues?

[russ]

1) With hydronic it is easy to have zones - that is what I have but using an air to water heat pump as the driver. We control the zone temps about like you describe. If I did it again I would drop the water system and use standard AC units - one master compressor unit outside and multiple fan coils in different zones.

2) What Sunking means is that it can not be done in a cost effective manner however you have indicated that is not the driving factor for you

3) Determine how many kW of heat you would require per day (with your architect). Use the insolation for your area to determine kW available on a typical day in the winter - I then multiply that by 85% to get rid of early and late hours when your harvest is nil for all practical concerns. Consider an efficiency of 50% for top line solar thermal panels. Insolation*0.85*0.50 gives you what you can expect to harvest on a typical day. That number and the kW required tell you about how many square meters of panel you would require on a typical day. Cloudy days the harvest is not much. You are talking about a lot of m2 of solar thermal panels

4) I have a green house - unheated - first year here so have some items I am trying - Everything is nice until it is freezing for a few days - my tomatoes were doing well until everything froze one day - actually two are trying to survive. Lettuce etc seem to just sit there holding on for better weather. Here all heat sources (LPG, diesel etc) are based on highway costs in the 10$ per range so auxiliary heating is not an attractive option. Next year I will simply empty the GH out in late December and start again in March. The only energy source that ir at all reasonable here is electricity at about 20 cents/kWh

5) Normally there is a reason something hasn't caught on - that it is not practical. If solar PV didn't have massive subsidies it would fall into that category. Here you see zero PV and lots of solar thermal - solar thermal can be cost effective.

6) Heating and, even more so, cooling of water with electricity is horribly costly - unless you don't care about water consumption and are willing to put in a cooling tower. Someone im the family better love water chemistry - cooling towers require constant baby sitting - all sorts of things love to grow in them.

7) We have all concrete and masonry construction so the insulation is on the outside - to keep the ambient hot or cold away from the thermal mass.

[Sunking]

Hotflas what I am telling you is you cannot have a battery sized only to last a few moments while a cloud passes by. With a hybrid system the power has to go out before it switches to battery. And when it does switch only critical loads will be kept alive as the ATS will operate and dumps the large loads like air conditioning. Otherwise you are going to need several tons of batteries.

[HotFlash]

Thanks, Russ! Some great info!

1. Specifically why would you drop your hydronic in favor of zoned heat pumps? Is it the equipment cost, or is it something about the daily operation?

2. Well, cost isn't exactly irrelevant, either. However, you can add into this equation that we can build much of what we need. Nobody has ever had confidence that I can build anything until I actually do it. However, I've fabricated things my entire life. I used to design and build half of my laboratory equipment, and today I design and build half of my photographic equipment. I often modify or re-purpose what I can buy inexpensively. If/when something gets beyond my skill level (e.g. welding), or if I don't have the equipment to build something (e.g. sheet metal fabrication tools), then I live in one of the largest ship building areas in the world, with neighbors all around me who work(ed) for the shipyard. With the hydronic system, I'm envisioning a lot of plumbing, Taco recirc pumps, and control systems, combined with the air handlers. For air handlers, I'm envisioning squirrel cage blowers, automotive oil coolers for cores, and a housing constructed by my local HVAC duct guy (who does amazing work as a subcontractor for the shipyard). Actually none of this is particularly expensive, except that it happens in multiples. We'll do one air handler first in the new master bedroom wing, keeping the old central forced air HVAC system intact. If that works as we expect, then we'll go through and replace the remainder of the system.

3. Thanks! And yes, a lot of m^2, but we'll have a pretty large rooftop. It's essentially a 1-story home.

4. We already considered the freezing. Our emergency heat will be from a wood-burning furnace. When the solar runs out, our greenhouse and home will get cold together. We'll fire up the wood furnace to heat both. The flue will ascend through the greenhouse and heat it passively, while the furnace unit will heat water. The greenhouse will also get some heat through its floor from the main structure. Our greenhouse will be geared mostly towards wintertime insolation, with a well insulated shed roof and a few skylights to throw some light on the plants during the summer. The main glazing will be multi-cell polycarbonate, R5 stuff. Thus, it won't lose heat so badly during a cold snap. In fact its passive temperature probably won't get below freezing anyway (in this climate), just from the bit of heat through the floor.

5. Sometimes solutions are unpopular because they're difficult to execute or because it's hard for a contractor to make a profit doing it. Still other times, they are unpopular because their construction falls across too many lines. A large thermal mass is the job of a building contractor, and solar thermal panels are the job of a solar contractor. I've not seen a contractor around here who both builds homes AND makes them green. Well... To be accurate, there is this one guy who builds passive solar homes with garden atriums and small PV rooftops, but he doesn't do anything apart from the little community he developed, and he's pretty set on how he does things (not willing to consider anything different). Interesting concepts, but a few problems in the implementation, largely worked out in ver. 2 of his basic design. http://gardenatriums.com/welcome.htm

6. Water consumption: Wow, we have more water here than you can believe! We have an artesian well in an aquifer that nobody else uses. We have 5 ft of saltwater at low tide at the end of our dock, with tidal action washing water in and out of a rather active river system. We have approx 48" of rainfall annually on a rather large roof on 1.5 acres of land, with steep runoff. (We'll be constructing a storm drainage system to mitigate erosion.) And then we have city water. We are billed for 2000 gal/mo, whether we want/use it or not. Anyway, I'm imagining a closed loop running 150 ft and back through the river water is as good a heat dump as I'll find. Algae and barnacles may well be the challenges to our system, but I suppose we can scale those everytime we scale our boat hulls.

7. Pretty much what I had in mind. As I'm currently planning it, the thermal mass tank for wintertime heating will be cast into a workshop basement constructed of ICF blocks. Extra insulation will be added around the tank. The tank will be lined with a high temperature plastic liner designed for that purpose. In the event it springs a leak, the water will dampen a concrete floor before it runs out the door into our storm drain system.

[HotFlash]

Sunking, I get your point. That said, my feet are already resting atop a device that could do the job in a PnP fashion -- an 800W double-conversion UPS made by TrippLite. There's literally no transfer time, as all of the computer equipment in our office runs constantly off of the inverter, while a charging circuit keeps the cells topped off. It would be easy to implement the same sort of thing on our rooftop. Specifically, the solar panels would charge the batteries through a charge controller, while an inverter simultaneously pulls off power to run the compressor. When the sun drops out for a predefined time, power is interrupted to the compressor and not restored until the sun reappears AND another predetermined time (maybe 5 min) has elapsed to prevent locking the compressor. I might have to cobble up some control equipment to do it, and I admit that efficiency is lost in the double conversion, but there's no reason it can't be done.

Again, all I'm trying to do is to flesh out the Plan B, should grid-tied PV be yanked away from us someday. Preparing for Plan B would involve making sure I have enough rooftop where I need it to support the larger number of PV panels that would be required to take our summertime cooling off-grid, as well as sizing our thermal mass tanks appropriately to provide for that capability. Then if the unthinkable ever happens, we'll have the option of modifying our system to continue using all of our expensive equipment.

My question still stands: Has anyone used a thermal mass to store "coolth" (hate the term), as an off-grid alternative to storing electricity? Water is cheaper than batteries and would seem more effective at storing large amounts of energy.

[Naptown]

My question still stands: Has anyone used a thermal mass to store "coolth" (hate the term), as an off-grid alternative to storing electricity? Water is cheaper than batteries and would seem more effective at storing large amounts of energy.

Yes this is done to some extent in large office buildings on rare occasions. However it is done for rate shifting in other words to cool and store a large amount of cold water during off peak hours for use during peak hours.
Now here is the problem with that in your case and with solar.
Your peak cooling needs will be during the peak sun hours. Unless you install enough PV to power your daytime cooling needs in addition to creating some storage for night use the system will not work.
If you indeed decide to go this route I would seriously consider partitioning your storage into smaller chunks and valving them to increase delta t for both heating and cooling Particularly for cooling. A 20,000 pound water storage will take a fairly large chiller to cool it down to where the AC will actually work (Evaporator coil temps on most ac units run about 40-45 degrees. Higher temperatures will do two things.
1 reduce the amount of dehumidification the system is capable of ( this is 2/3 the battle with AC)
2 reduce the amount of actual cooling you will get
There is actually a lot to consider in an undertaking like this more than just storage and distribution.

[Sunking]

Sunking, I get your point. That said, my feet are already resting atop a device that could do the job in a PnP fashion -- an 800W double-conversion UPS made by TrippLite. There's literally no transfer time, as all of the computer equipment in our office runs constantly off of the inverter, while a charging circuit keeps the cells topped off.

You are talking about a dual conversion UPS, and yes I know all about them as I have designed the real things for data centers and telephone companies where they use 2 MW UPS. What you have in the office is a toy compared to what you are asking for to power up a 5 ton Water Chiller. That is the part you do not understand.

Can it be done? Yes, I do it every day. Can you afford it on the scale you mentioned? I doubt it seriously. Where is the power coming from on cloudy days and at night?

For a 5 ton water chiller the batteries to be able to drive a industrial grade 10 Kw True Sine Wave inverter of say 96 volt batteries requires a minimum 400 AH string @ 96 volts. That battery weighs in at 3000 pounds and cost $9000 plus about another $20K for an industrial grade 10 KW inverter. But that is only the start of your problem. How are you going to supply power to this non existent Solar UPS. Those $9000 batteries only give you about 15 minutes of power.

Check out what a 10 KW UPS cost like this one and it includes1 battery for 5 minutes back up.