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Hartland Cohousing Project System Design - Memo 1
by Marc Rosenbaum, P.E.
Loads
Water
Solar Heating
Solar Hot Water
Wood Heating
Masonry Heaters
Solar Electricity (PV)
Metering and Fuel Usage
Composting of Vegetable Waste
Recycling of Solid Waste
4/10/98
From: Marc Rosenbaum
Subject: Systems Memo 1
This memo is written as a follow-up to the Design Committee meeting on 4/3/98. It feels appropriate to get into more detail so that these decisions can be made in a timely manner as site planning gets underway. This memo will cover aspects of the potable water system, heating systems fueled by solar energy and cordwood, solar hot water, solar electricity, graywater, and composting. Recognize that this is early in the design process and actual loads are far from being calculated. Agricultural needs are not covered here (although remember that a methane digester should be included in the site planning, adjacent to where the animals are bedded.)
To do a more accurate job of picking systems, loads must be estimated. There is information about what is typically required in homes for potable water, space heating, domestic water heating, cooking, and electricity. I will be making assumptions that should be tested as we proceed about how community members' behavior will differ from typical. It is fairly easy to know that the homes will require less heat, but much more difficult to guess whether people will use less potable water, hot water, and electricity.
I am not a water systems engineer, so I am not familiar with how these systems are designed. What follows is based on my understanding of the basic physics and how pumps, etc., work. The assumption here is that the state will require a drilled well of moderate capacity (say 10-20 gallons per minute (gpm)) and also a tank which can store some thousands of gallons (I believe I heard at least one day's worth of projected use.) We don't know as yet how the state will regard the community's projected use if dry toilets are used - we currently understand that, with water-conserving fixtures, a figure of 135 gallons per day per bedroom is used. Say daily usage is 6000 gallons.
Power is used to pump water from the well to the storage tank, because the water must be lifted. This pump is called a submersible pump and is located near the bottom of the well. Submersible pumps need to be capable of lifting the water from the bottom to the top of the well, even though what is called the static level (the equilibrium water level in the well when no water is being pumped out) may be very close to the ground surface. This is because during periods of high usage the well may be drawn down far below the static level. With a storage tank, this effect may be mitigated, but the pump will likely have the capability anyway, because it is probably not possible to buy a pump that doesn't have the capability (assuming the well is within reasonable depth.) Unfortunately, submersible pumps don't do very well at "unloading" as the work they are required to do (i.e., lifting the water) decreases due to a high static level, so they require a fair amount of power. The pump/motor combination is unlikely to have a combined efficiency exceeding 50%. I am looking into whether there are more efficient submersible pumps designed for solar power in the size range we are likely to need - the ones I know about are quite small. It may turn out that a small pump running almost constantly is more efficient than a larger one which runs only a few minutes per hour. 6000 gpd is a little over 4 gpm continuous, which is even lower than the flowrate of my home well pump. I suspect that we will want utility power or generator power back-up for the winter months if the primary pumping energy comes from photovoltaics (PV).
Once the water is in the storage tank, it can get to the homes either by gravity or by pumping it. Recognizing that I don't know what the state may require, it seems very desirable to have gravity flow to the homes. Some old farmhouses had gravity fed water from springs high above the house, and the difference in height between the spring and the house was sufficient to provide enough pressure to service the water-using fixtures in the home. Every foot of elevation difference can yield about 0.4 pounds per square inch (psi) of pressure. Typical modern house water pressure is 25-50 psi, so in a conventional home the level of the water source (in our case the bottom of the storage tank, or whatever is the minimum water level in the tank) would need to be over 60 feet above the level of the highest user in the house (typically a second floor showerhead), so in practice the tank bottom would need to be about 80-100 feet above the grade at the house. This appears unlikely on this site, based on looking at the places the houses might go and the places a water tank light be located.
If the water makes it to the house under gravity feed but with insufficient pressure to run the appliances, then a pressure tank and pump could be installed to raise the pressure to the normal levels. We need to find out the way a small community water system does this, because it may be typical to have a pump house at the storage tank which pressurizes the supply line to the houses, just as it is done in a city water system. But this would make gravity feed to the houses difficult.
One unusual approach would be to design the houses to use much lower water pressure and see if gravity feed could do the job without any secondary pumping. This uses less power. Showerheads are available for this type of systems, and I don't know about faucets. Washers and dishwashers may be more difficult (will there be dishwashers in the individual homes?) because they may be set up to work with a minimum pressure that is higher than what we may have to work with. This could be handled more easily for the washers if they only exist in a few locations, because each laundry pod could have a pump/pressure tank just for the washer. Alternatively, the washers may permit modification to work with low pressure water. Lawn sprinklers are definitely out on this system! Please understand that any load which required normal pressure could have a pump/pressure tank, but once every house needs a pump/pressure tank, the impetus to design the low pressure system shrinks, because the high pressure is available. I'd like input from the group about whether this gravity feed/low pressure system design idea should be pursued - first thing to do is to check in with the water system designer to see if it is legal. It would be a nice feature in a major power outage to have the water system remain serviceable. As a minimum, I suggest that the Common House be designed so water is always available.
I am assuming that the buildings will be built to a better-than-average standard, especially as regards airtightness, but I am not assuming as yet that the extra money will be available to superinsulate them (primary differences - wall thickness goes from 6 to 10-12 inches, roof insulation is increased, a higher degree of airtightness is achieved, the best windows are used, heat recovery is implemented on the ventilation air.)
The three solar heating methods, in order of effectiveness at reducing back-up heat requirements, are suntempering, passive solar, and active solar. Suntempering is a strategy in which the house is oriented towards the south, and most of the window area, say 2/3, is placed on the south side. Total window area is constrained by the fact that no thermal mass (typically masonry floors or walls, less frequently water containers) is added to the house to absorb excess solar heat which comes in through the south glass. This glass area limit is about 7-8% of floor area - above this thermal mass must be added, at some cost. Most houses people describe as passive solar are in fact just suntempered, as they have either no added mass or inadequate thermal mass to store the excess heat, so they overheat on a sunny winter day.
When mass is added, the window area can increase, to 12-13% of floor area. Note that if masonry is used as the mass, you need 5-6 square feet of mass which gets direct sun for every square foot of glass added over the suntempered case. This type of passive solar heating system is called direct gain, because the sun's heat comes directly into the space. (An example of indirect gain is a sunspace). Direct gain is usually the most cost-effective passive solar strategy, as it is the least costly. If alternative construction methods are used (masonry blocks, straw bale, etc.), the thermal mass is already present in the structure, and making a direct gain house is easier than when masonry needs to be added to a wood-framed home.
Active solar heating can make the largest dent in the heating load, because a lot more collection area can be installed without having to provide storage in the living space. In addition, unlike a direct gain home, which loses heat back through the "collector" at night or on cloudy days, an active solar home isolates the storage and collection from the living space. Temperature swings in the space are much smaller. The other major advantage is that heat can be stored at temperatures much higher than would be possible in the living space, so it is easy to get multiple day storage from a single good sunny day. Finally, these systems also make domestic hot water.
Efficiency of active systems is improved by using low temperature heating distribution systems such as radiant floor heat. This also increases cost over a baseline home.
The last active house I designed (in Hanover) has a 360 square foot collector on the roof and a 1200 gallon tank in the basement. Because the house is extremely superinsulated, and suntempered, the solar system provides close to 80% of the heating and hot water load. The solar system cost was about $10,000. The value of the heat displaced depends on the fuel cost - it displaces about $150 of cordwood or fuel oil at current prices, or $700 of electric resistance heat. Once most of my clients hear the cost vs. savings numbers, active solar is ruled out. A house needs to be superinsulated in this climate to consider active solar, and given the budget pressure on this project, I don't recommend active solar as a strategy.
To illustrate the impacts of these strategies, here are some numbers from a paper presented a few years ago, in which a small ranch house located in Hartford, CT was modeled. The base house is similar to what I would guess is the minimum level which the group would choose - well-insulated, low-e windows, good airsealing. Suntempering that house saves 12%, and changing it to direct gain passive solar saves 23%. Superinsulating it instead saves 39%, so you can see the relative value of superinsulation vs. the solar strategies. Combining superinsulation with suntempering saves 46%, and combining superinsulation with direct gain saves 58%. I would guess that a combination of suntempering and some level of building efficiency improvement will be where this group will end up, especially if wood, renewable and inexpensive, is chosen as the primary space heating fuel.
Taking a typical small house to the minimum level of improvement I envision probably costs about $1500-2000. Superinsulation probably adds another $2500. Please note these are budgetary only!
Assuming typical but conserving hot water loads, an area of 40-70 square feet of collector area should be accommodated per household. This needs to face within 45 degrees of south and on a fairly steep roof pitch -10 or 12 in 12. The lower number is appropriate for backing the solar up with wood heated hot water, so that the solar system is doing the job in the warmer half of the year. Where the laundries get located will affect sizing somewhat. Solar system design will be significantly affected by heating system choice - if a central system heating several units is chosen, then tying the solar hot water in with that will make sense. One consideration will be distance between units. The distance between the collectors and the storage tank should be minimized, since the hottest water travels between them. One central tank per pod may make sense, but this will need to be balanced against the time it takes for hot water to reach an end user (longer pipes, more time.) As an example, Ecovillage at Ithaca opted for a system that put two gas boilers together to serve the heat and hot water needs for 8 units, but each unit had its own hot water tank, heated by the central boilers.
I estimate the cost of solar hot water to be $2000-3000 per unit.
The size of the solar hot water system could be reduced by reclaiming heat from the graywater leaving the units, especially if the major use is showers. In order to use the graywater heat recovery device which makes the most sense, the graywater line must leave the house well below grade, as these units are 5-6 feet tall, and go into the drain line below the lowest floor in the house (assuming a shower on the first floor.) These are only a few hundred dollars each, and they can save 40% of hot water energy usage.
A slightly different graywater heat recovery device might be considered in the laundries, since hot water usage is not simultaneous with graywater discharge. Removing the laundries from the homes makes virtually all the hot water usage simultaneous if showers are used instead of baths.
Graywater treatment will optimally be accomplished with a strategy that uses the nutrients. A greenhouse producing non-edible plants would be a great match, providing a living for a community member while cleaning the graywater stream. Then a decision will need to be made about the relative value of the heat in the graywater, and whether it belongs in the homes or in the greenhouse (helping boost plant growth). If the greenhouse is far away from the homes, the heat may as well be recovered in the homes, because otherwise it will mostly end up in the ground around the piping.
The three wood heating options I see are: a central (for 4-8 units) wood-fired gasifier boiler supplying heat and hot water, woodstoves in each unit, or masonry heaters.
The wood boiler envisioned is the same brand which Dana has in her house presently. These burn wood at very high temperatures, resulting in efficiencies of 75-80% and low emissions. They can be bought with oil or gas back-up in sizes up to 140,000 Btu/hour, or in a wood-only version at 200,000 Btu/hour. These are way too big for a efficient home (I expect a 1200 square foot unit to require 16-22,000 Btu/hour) but just right for a grouping of homes. Sizing will depend on hot water requirements, but the wood-only unit could probably serve as much as 8 units, and the combo unit 4 or 5 units. The wood-only unit would be backed-up by a separate fossil fuel boiler, which might be a better choice as it could serve as an efficient summer back-up for hot water, too.
Advantages of the boiler over individual woodburning appliances are: keeps the wood burning/storage mess out of the houses, fewer chimneys, ability to provide hot water, shared responsibility for feeding the wood burner, ability to more finely control temperature in the spaces (because the boiler is connected to conventional baseboard heat or in-floor radiant heat) and higher efficiency than woodstoves. Another potential advantage is that community members that don't want to or are unable to burn wood can enjoy the benefits without having to have woodburning equipment in their own space. Disadvantages include: requires electricity to run both the boiler and the heat distribution systems, makes some noise, there is some heat loss in distribution to units, especially if they are separated enough to require underground piping, can't cozy up to a hot thing in the living space. The units should either be attached (best) or close enough to connect the basements with a 20 feet piece of underground sewer conduit so that pipes (and wires) can be run from house to house.
One of these units with a gas-fired boiler for back-up probably costs around $10,000 installed. The distribution system will likely add about $2-3,000 per unit (really rough guess, depends on many choices along the way).
Woodstoves are familiar to most people. They are much cleaner than they used to be, many of them using a catalytic converter. They don't make as much creosote. They are as good as 70% efficient at burning wood. They need no power and they are quiet (except when they back-puff!) You can cook on some of them in a pinch. They require more frequent feeding than a boiler or masonry heater, and they don't heat hot water (the older, pre-EPA approved stoves could heat water - I have this in my house - but they can't keep the fire clean burning if there is a heat exchanger in the stove. Sigh.) There is no zoning of heat and temperature swings are higher than in centrally heated homes. Most people (and the bank, if they are involved) will want to see back-up heat also. With a chimney and hearth, and with a cheap through-the-wall gas back-up heater, this will probably cost $4-5,000 per unit, depending on whether the chimney is visible and therefore brick, etc.
Originally from northern European, masonry heaters have been used in North America for over 20 years. The principle is that the "stove" is massive, so the fire can be burned flat out (meaning clean and efficient) and the heat is released gradually, principally by radiation (gosh, I got principle and principal in the same sentence - anyone care to try compliment and complement?). These units were typically room heaters in Europe and were not intended to heat a whole house. In a small, open plan, efficient house, they can do the trick.
Masonry heaters are expensive, because even the kits require a fair amount of skilled masonry on site, plus a foundation for the heater. Figure $10,000 installed. A few of the products available allow the heating of hot water, and can incorporate a bake oven and even a cooktop! Heat, hot water, and cooking from a locally harvested, renewable fuel. Greg Allen in Canada has suggested that a small Stirling engine could be incorporated into the back of the firebox to also generate electricity, but you can't buy that as a product today. Advantages of masonry heaters are: cleaner and more efficient than woodstoves, no power required, quiet, cozy to be near, long, slow release of heat, can heat hot water, can permit cooking, only need firing once or twice a day, no creosote issues, "natural" feeling because of low-temperature radiant heating. Disadvantages are: cost, space required in the home, wood needs to be brought into the living space, can't turn one off once you light it (e.g., you thought it was supposed to be cloudy today but the sun came out a half hour after you lit it off.)
Both woodstoves and masonry heaters allow any amount of space between units, since nothing is shared. Solar hot water systems would be individual.
Calculating loads gets important here, because once a decision is made to use expensive PV, it really pays to select very efficient lighting, refrigeration, computers, printers, etc. A conservative estimate is that a household may use about 2000 kWh per year (check your electric bills to see where you are right now!) with efficient equipment and a conserving mentality, but without heroic investments such as $2300 SunFrost refrigerators. I've recommended that the equivalent of 200 square feet of contiguous, south-facing, 312/12 roof area be planned per household. Electricity is easy to move around, so this area need not be directly on the houses. I am recommending a grid connection, so large amounts of batteries and a back-up generator aren't needed, but some storage makes sense to carry critical needs during power outages.
A decision will need to be made about whether the group wants to meter the energy use to each unit, or decide that they would prefer to spend the metering cost (could be upwards of $500 per unit) somewhere else. If wood is used, an estimate of wood to supply heat and hot water for a 1200 ft2 unit is 1-1/2 to 1-3/4 cords/year. Is it worth metering if the difference between units comes to $50-75/year? A allocation system could be set up based on area of units (heat) and number of occupants (hot water). It has been pointed out that having feedback on usage is a good thing to help shape behavior, and it is useful to realize that this does have a cost.
This could be done in individual compost heaps, or in more sophisticated composters which control aeration, moisture content, and temperature much more closely. These units produce a good product and would be shared amongst multiple users. The other option is to feed this waste to chickens - some figuring needs to be done to compare the amount of vegetable waste generated by 20 households to the anticipated chicken population!
This bears more thought by the community - should collection and storage be at individual units, at the "pod" level if that is how houses are organized, or at the community level, with members responsible for getting their recyclables to a central location?
- Marc Rosenbaum
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