Going Completely Off the Grid
Q: We are kiwi's all the way over in New Zealand.
And I was hoping to pick your brains about the best way forward to go completely off grid.
We already have solar water, doubleglazing and good levels of insulation. But would like to add a windturbine and PV panels to the equations in the hope that we don't have to give up the day to day luxury's of life. Can this be done and what is the best way to go about it? I want my cake and eat it like most people do!
Is is worth getting a professional in to do a full feasablity study covering what our usage is and when and if we should use wind, solar or a combination?
Have you got exactly what you wanted or would you do things differently if you did it over again?
If you wanted to update your system what sort of new eco technology would you love to have? Believe it or not, off the grid is still quite a new thing in NZ and we are still in the dark ages where thats concerned.
Here is hoping you have some good advise.
Regards
Ruth
A: Hi Ruth,
Everything you need to do your own PV & wind evaluation and design is online these days.
Nearly every company that sells inverters for PV systems has load design sheets posted so you can just fill out the electric usage, then go into the sunshine tables (see www.nrel.gov) for your area, get those numbers for each month, then go to the panel selection, the number of panels needed –and there you are.
Some company names that you can search for include: Evergreen (for panels), and Outback, Fronius, Sunny Boy, etc. for inverters.
You will benefit from getting HOME POWER magazine and reading back issues too.
Installing is another matter, for which you need a professional on the job according to your local electric codes.
My house is 100% solar heated but I am on the grid for electric.
I will add a PV design module to my website – www.crestonesolarschool.com - pretty soon.
Paul
Installed cost of solar thermal, flat plate and evacuated tubes, per unit of energy
Q: I am trying to nail down the installed cost of solar thermal, flat plate and evacuated tubes, per unit of energy. Do panels have a rated capacity like they do with solar electric systems? Is it possible to say that solar thermal costs $1 per watt of rated capacity like you can with solar electric ($6-8 per watt)?
A: Yes, it is possible to express solar thermal in $/watt...
Solar thermal panels --4x8 size— are rated around 30,000 BTU per clear day output (that’s one fact)
Installed cost of PV runs around 10 $/watt for a grid tie (no batteries) which translates to about 100 $/sq ft of panel whereas solar thermal installed cost also runs around 100 $/sq ft, but it collects thermal energy and converts sunlight to heat at higher efficiency than PV, and this can be converted to about $2.30/watt for thermal solar (as opposed to $10/watt for PV-solar electric).
This difference is purely based on the conversion efficiency between the two types of panels all converted into watts (it does not take into consideration the different energy quality between heat & electricity –you cannot run your drill on solar heat)
Active Solar Heating System
Q: Don’t you need an air vent at the top of the collector array?
A: No, it is a drain-back system.
Q: How can I tell if my Solar Hot Water system is working?
A:--- some basic verification observations for the Homeowner.
3 easy BASIC VERIFICATION steps that anyone can do with minimal training.
The immediate and simple verification test that I use is to go to the home at midday on a sunny day.
(this seems to be essential for the simple verification test below):
1. Determine if collector pump is running, and if the heat exchanger pump (if there is one) is running.
TO PERFORM --BASIC VERIFICATION TEST--:
2. Measure the temperature difference (delta T) between the collector feed pipe and the pipe coming back from the collectors.
(To locate these two pipes -usually these two pipes are going up thru the ceiling;
To measure temps, peel back the insulation and use an infrared thermometer,
or just feel the bare pipes with your hand if not too hot –don’t burn your hand)
This temperature difference should be around 10F degrees (8-15F degrees is a good range), and will verify if the system
is collecting solar heat at mid day.
If there is little or no temp difference, the system is not working; if the temp difference is very large, the system in not efficient.
3. You can then check for shading & orientation from the ground, because at mid day the sun is in the south, and you can eyeball the orientation, and then, if it’s summer, visualize the low winter sun arc ... Do you see any obstructions?
(Homework: get very familiar with the summer & winter sun paths & angles across the daily sky before site visit)
(minimal training needed: learn how to identify the pumps, and which pipes are going to & from the collectors)
-Paul Shippee
Q: My solar designer friend cautioned me about summer overheating. Will we need some type of solar heat mitigation for summer months?
A: What exactly do you mean by summer heat mitigation?
Q: Several people, Curtis Smith of Solar Design Depot included, feel we may have too much heat from the thermal panels during the warmer months making the house too warm. I don't want to cover the thermal panels---if I don't cover them, will things be okay without some type of solar heat mitigation?---i.e. typical ones----dumping it into a hot tub or, as Curtis suggested, running pipes underneath the floor and sending extra heat into the earth, etc.
A: OK, I think I see some confusion here about two different types of solar heating systems. Your system is a drain-back type of system. Now I realize those people talking to you about overheating your home in summer are assuming that you have a glycol antifreeze type of system.
Let me explain the difference, and why your drain-back system will not pose a summer overheating problem. Antifreeze systems, otherwise known as closed loop systems, are always filled with liquid under pressure. And they are vulnerable to boiling in the summer, --like whenever the circulation pump turns off, say during maintenance or a power failure, and the sun is shining. This is called the stagnant or stall condition. Because the fluid is not moving or circulating, it creates high temperatures which can, in turn, create a high pressure explosive condition and a corrosive environment for the glycol antifreeze mixture.
Because of this, there is a need to keep the circulation pump running regardless of whether heat is needed in the house or not. And yes, the need to run the circulation pump all the time during sunshine hours can lead to overheating during the summer. This naturally brings up the consideration of where to “dump” this extra heat, and how to keep it moving and circulating. Added to this is the possibility of a pump or power failure where the stagnant fluid in the collectors will reach stall temperatures of 400F, thus causing those high pressures and a corrosion environment. These can be major drawbacks. Other drawbacks seem to be; the need to maintain the glycol effectiveness into the future, the toxic potential of antifreeze chemicals, and the lower heat capacity and less efficient heat transferof antifreeze mixtures compared to plain water.
However, drain-back systems utilize plain water and are free of those drawbacks as well as the summer overheating problem. Why is that? Because whenever the solar collector pump turns off, the plain water drains back to a reservoir tank beneath the insulated roof or house ceiling. Now there is only air in the collectors, and there is no freeze hazard. In summer the solar pump can easily be turned off by a room thermostat,a domestic hot water tank aquastat, or a power failure. No problem. If the system is designed and installed correctly (a big “if”), all the water drains back into the warm house environment. The collector piping is full of air only. Overheating still occurs but solar collectors full of hot air and some temperature sensors, are built to take the excessive heat. Caution: during a power failure, provision must be made for the system to drain back under any conceivable (or inconceivable) potential flow blockages, such as; closed valves, anti-gravity pipe runs, or overfilling the system.
So this is why your solar designer friend cautioned you about summer overheating. He didn’t realize you have a drain-back system.
In addition to the natural protections that drain-back solar thermal collector systems provide against overheating and freeze hazards, described above, they are often placed at a high 60 degree angle to the horizontal – like when employed for winter space heating to favor the low sun angles. This presents a smaller collector profile to the very high summer sun, thus reducing summer solar heat gain. Of course, this geometric foreshortening of the solar target applies to glycol systems as well.
In conclusion, in order to protect against summer over heating there is no need to cover your solar collectors, or to install hot tubs or earth pipes to “dump” unwanted heat.That’s good news, isn’t it?!
-Paul Shippee
Thermal Mass for Passive Solar Heating Systems
Question asked: How much thermal mass heat storage is actually required for a high-performance passive solar home system that approaches 100% solar heating fraction?
Paul’s answer: The function of heavy thermal mass materials (water or earth) placed inside the building is to moderate day-night temperature swings, and to store daytime solar heat for slow release on cold winter nights. However, most people are skeptical and are surprised to learn how much thermal mass heat storage is actually required for a high-performance passive solar home system that approaches 100% solar heating fraction. In general, water stores twice as much heat as earth materials for each degree (F) of temperature rise.
Passive system types are often defined by how the storage is arranged. So, in addition to the actual volume of storage, other aspects like location of the storage materials and the area of their surfaces are important.
Question asked: What can you tell us about Water Walls, Trombe Walls & Direct Gain?
Paul’s answer: Storage is the heart of the system as it collects, stores and distributes the heat. Although it does this inside the living space, the air temperature is not always the same as the storage material. The proper goal of storage design - volume, location and surface area - is to serve the comfort of the occupant by reducing temperature swings.
The partial decoupling of thermal storage effects is intentional; and is accomplished by different principles for each of the 4 passive system types:
Water wall - large mass
Trombe wall - time lag
Direct gain - large surface area
Sunspace - isolated space
Water Walls
Storage sizing for water walls, since there is no time lag effect, should tend toward large masses. Use a storage density value of 80 - 120 pounds of water for each square foot of
collector area in order to keep the maximum clear day temperature rise under 12F.
A general rule for sizing might be: You can't have too much. thermal mass! But you may find limits imposed by cost space, and the trouble to heat it up if it chills.
The area of hypothetical 80 pound density water mass may be equal to the glass area. The thickness of it is 80/62.4 = 1.28 ft. Or the same mass may be split so that half of it is lit by the morning sun and half by the afternoon, still giving a 12F rise.
This curve shows the relation between water storage volume (or weight) and its clear day temperature rise (for 1000 BTU/ft2/day solar collection). 60 lb/sq. ft. collection gives a temperature rise of 1000 BTU/60 lb. = l6.7F. Higher rises (less volume) would indicate further decoupling of the mass from the space by putting the containers in a closet. It is difficult to justify anything over 80 lb. unless larger containers are cheaper. The law of diminishing returns (again) begins to level off the benefits. Also, very low temperature swings hurt the distribution heat transfer into the living space. So, a range of desirable storage volume values for exposed water walls is defined by daytime overheating (storage too warm) and difficult night time heating (storage not warm enough). The lower limit is less serious because it is somewhat self-regulating -- as space temperatures fall more heat flows out of storage.
Trombe Wall (Masonry Walls)
Storage sizing for masonry walls is a different matter. Daytime overheating is protected by the time lag of the wall (if it is non-vented). It has been previously recommended for vertical double glazed mass wall systems to use a 12-inch thick concrete wall with night insulation, or 16-inch thick without night insulation. The equivalent water mass of a 16-inch thick concrete wall is:
1.33 ft x 1 ft2 x l40 lb/ft3 x 0.2 BTU/lb/F = 37 BTU/F or 37 lb. water.
This thick wall is much smaller mass than any recommended water wall, yet is stable and comfortable. This is because the combination of its conductivity and thickness produce the correct time lag for phasing daytime winter sun, night time cold, and living space comfort.
Direct Gain
Sizing for direct gain systems depends on having a large enough storage material area (if it is masonry) for the sun to play on throughout the day. The area, in contrast to mass walls, should be several times larger than the glass area. This is necessary to reduce temperature swings because the sun shines on the same side of the masonry mass as the living space. The sun should not stay long on one section because the outer layers of concrete insulate the inner layers. In this case the conductivity is too slow to alleviate daytime overheating of the air.
Methods of spreading the sunshine over large areas of masonry wall mass are by providing patches of sunlight, by diffusing the light, or by reflecting it from floors and other surfaces. Reflections may bring interesting natural lighting effects. The color of surfaces should be light to help distribute the energy over these surfaces. If the same 16-inch thick mass wall is distributed over 6 times the area in a direct gain system, the wall only has to be 2 ½ inches thick. The performance of a thin wall of large area has not yet been demonstrated. It may be difficult to play the daily energy uniformly over this large area in practice.
Mazria and others have shown, in a computer study, that space air temperature swings may be reduced from 41F to 13F by increasing storage area from 1.5 to 9 times the glass area. The walls are 8" thick and 4" thick respectively, so more mass is used in the distributed area system. Other benefits were higher daily minimum temperatures and no daytime ventilation required. Cloudy day storage analysis was not done. Adobe walls with a low conductivity of 3.6 BTU/hr/ft2/F gave the highest room air temperatures.
If the sun does not shine on a part of the storage mass, then it is called secondary storage. The question arises as to its effectiveness. If the material has good conductive properties (e.g., concrete) and high thermal admittance (i.e., no carpets), then excess heat may be transferred into the material from the warm air. It is doubtful this will take place very rapidly under any conditions. A vented Trombe wall needs some place to put the 30% heat delivered by thermosiphoning air, and large areas of secondary storage will certainly help.
Two other advantages of secondary storage are for heating when the primary store is depleted and for summer cooling. The rate of falling storage temperatures will level off considerably when a large mass of secondary storage comes into play. This shock absorber effect happens when the primary storage temperature approaches the temperature of the secondary storage and the air temperature.
Direct gain and mass wall combinations
Direct gain and mass wall combinations may be made by putting windows in the mass wall. This allows some day lighting and provides for a view and some relief from the imposing requirements of solar thermal storage. If the mass wall is masonry and it is vented, any more daytime heat from direct gain may be too much. If the direct gain openings in the mass wall are the only source of daytime heat, then they should be properly sized. A recommended rule of thumb is: less than 10% of the floor area if there is no other storage.
The maximum area may be quickly calculated for the average January heating day. Adjustments should be made according to whether primary storage is water or masonry. The water wall sits in the room at about 10F above air temperatures and is heating the room. The masonry wall is usually a good deal cooler, perhaps at comfortable room air temperature, until very late in the afternoon.
Sunspace
Storage sizing for sunspace designs follows the same principles as for direct gain and mass wall. But it offers the additional feature of drawing off excessive daytime heat from the sunspace in order to cool it, and to store it during the evenings (or use it if needed during the day -- but this need should be provided by direct gain openings). The best way to store the excess greenhouse heat is to blow hot air through a pancake rock bed under the concrete floors. There are several variations on this theme, such as water containers in a crawl-space. If this is done correctly, it may only store 30% of the total heat collected, but due to the low temperatures (90F max.) and thick slab, the heat doesn't flow into the rooms until after the primary storage has cooled down. Warm floors add comfort and the stored solar heat is spread out over greater low temperature areas and over a longer time period.
The size of the rock bed is figured for about a 10F temperature rise absorbing 30% of the total available collection.
If a 400 sq. ft. collector is used, the volume of rocks is:
.30 x 400 ft2 x 1000 BTU / ft2 /day x 
= 600 cu. Ft .
This would cover a 25 ft x 24 ft subfloor 1 ft deep.
The air path should leave the sunspace at the top, enter the rock bed along the north edge via a plenum and return to the floor of the greenhouse. Care must be taken not to develop too much air flow pressure on the fan. A 25 sq. ft. plenum face area and 24 ft. path length is not excessive, but the static head should be determined.
Question asked: What are the Thermal Mass Sizing "Rules of Thumb" for Passive Solar Heating Systems?
Paul’s answer: Sizing Rules of Thumb (for specific climate 6000 DD, 65% sun)
-
Building Load 4 - 8 BTU/DD/ft2 floor area
-
Collector Glass Area 20% floor area
-
Collector Glass Area Supplies average January day load
-
Collector Glass Area
Night Insulation 1.0 sq. ft. coll.
BTU/hr/F
No Night Insulation 2.0 "
-
Solar Participation 60% or more of total heating load
-
Storage Density
Water Wall 60 - 150 lb H2O/ft2 coll.
Masonry Wall 35 - 75 lb H2O /ft2 coll. (water equivalent)
-
Night Insulation
Sunspace NO
Direct gain YES R10
Mass Wall YES R10
-
Direct Gain Windows in Mass Wall 10% floor area or less
or 30% of south glass
-
Air Temperature 24 Hr. Swing 6F Normal
12F Extreme
-
Storage Surface Area
Mass Wall
70% glass area
Direct Gain
3 - 6 times glass area
-
East, West, and North Windows 5 - 8% floor area
Ventilation for Passive Solar Addition
Question asked: My wife and I are adding a solar addition to our house - with a slab, south-facing windows, and tiled floor & wall facing the windows. At this point we're trying to figure out what venting to use for bringing air into cooler parts of the house. Can you suggest any calculators for how much vent area we'd need to allow for convective air flows?
Paul’s answer: I have used a fan to assist convection in the past for deep N-S homes (350cfm per 200 sq ft of glass -with thick water wall in sunspace to also absorb solar heat for nighttime).
You can see a poster with performance monitoring results on this Nederland, Cold Springs attached greenhouse on my website: www.crestonesolarschool.com/solar_homes.html . (You will find the greenhouse with link to poster at bottom of page.)
Free convection areas I have had success with are about:
One sq ft high outlet/supply vent per 40 sq ft of south non-low e glass.
Same on return vents (these are minimums, use 1.5 sq ft if you can).
Ask Paul He will answer you personally and the Q & A will be posted at a later date.
