 |
|
|
|
|
|
 |
|
Hartland Cohousing Project System Design - Memo 3
by Marc Rosenbaum, P.E.
8/14/98
From: Marc Rosenbaum
Subject: Systems Memo 3
This memo is a response to requests for systems evolution thinking now that there is more clarity about housing decisions. Without an actual site plan and house/Common House designs, this is still provisional. Much related information is in my Hartland Systems 1 memo (4/10/98) and Hartland Systems 2 memo (6/11/98) - apparently I write these every two months. This memo is divided into Nutrient Recycling, Water Supply, Thermal Loads/Envelope, Advanced House Options, Common House, and Future Opportunities.
The criteria are: to preserve nutrient value by getting nutrients to the active root zone of beneficial plants, to prevent groundwater pollution caused by conventional "waste disposal" systems, and to ensure health by keeping people and animals separated from pathogenic material. Nutrients in the flows exiting a typical dwelling are roughly 80% in urine, and about 10% each in graywater and feces.
There are a number of alternatives for treating these products - small packaged sewage treatment plants, solar aquatic waste treatment (Living Machines is one product), constructed wetlands, sand filters, and composting systems. All but the composting systems first mix feces and urine with water. There are several disadvantages to these systems:
- A substantial increase in water use in the buildings, used as flush water for toilets. In areas of limited water supply, this may have a significant impact. In almost all cases, energy has been used to collect or pump the water, and in some cases to store, filter and/or purify the water used. This is true even in systems where rainwater or recycled graywater is used as flush water, since the storage capacity must be provided, as well as filters and energy for pumping.
- Increased system complexity, due to pumps, valves, controls, filters, heaters, etc., which are often part of the system and must be maintained.
- Substantial energy use, depending on the system type. Energy may be used for pumping, mixing, blowers/aeration, or heating.
- The nutrient value is not available for agricultural purposes, and in some cases, is not removed from the waste stream (e.g., constructed wetlands do not remove nitrates unless substantially oversized or supplemented with aeration.)
- Other disadvantages may include: the need for an enclosure for machinery or aquatic systems; quantity of relatively flat land area needed to accommodate the system.
In contrast to the above, composting toilets are contained within the building, have few moving parts, use little power (one model uses a 12 watt fan only), conserve water, keep nutrients out of the groundwater and permit their use as fertilizer. Different products vary in their complexity, power use, and ease of use, but the better products are acceptable on all three counts. Composters consist of a large composting tank, located below the lowest level with a toilet, and dry toilets connected by straight vertical chutes to the tank. Micro-flush toilets can also be used, with some penalties (see below), and these don't need to be directly above the tank, although they need to be relatively close. One potential drawback to composters is that adding a wing later to the house which contains a second bathroom may necessitate adding a second unit (expensive!), because it is too far from the existing composting tank. At Island Cohousing, we set an initial design constraint that the 2 bedroom unit would be designed so that it could grow into the 3 and 4 bedroom units, and that a closet (or optional half bath) in the 2 BR could become a full bath when the third bedroom was added. This is a significant constraint (although John Abrams pulled it off beautifully.)
If composting toilets are used, the end products are compost (similar to humus) and compost liquid fertilizer (CLF). Most of the nutrient value is in the CLF. The amount generated depends on the users' habits, the amount of ventilation air flow through the composter, the temperature and relative humidity of the air flow, and design factors particular to each composting toilet product. The CLF produced can vary between none and about 100 gallons/year per person. One composter manufacturer, Clivus Multrum, has had CLF from various installations tested extensively and has determined that this liquid usually has been treated as it passes through the composter to the extent that it has fecal coliform counts below 200/100ml, the EPA standard for water which is acceptable for swimming. The CLF also has little odor, unlike raw urine. It has salt content, which indicates the need for some dilution when applied to planted areas. Clivus reports an average fertilizer value (N/P/K) of 0.23/0.11/0.17.
Most state regulators are reluctant to accept that the CLF is pathogen-free. MA allows (by right) CLF to be disposed of in a conventional septic system or hauled away by a licensed septage hauler. Currently, there is interest in other solutions at the MA regulatory agency, so Island Cohousing will be proposing a different end use for the CLF. One option for CLF is to collect it and apply it periodically to land areas which grow non-edible plants (ornamentals, orchards, sugarbush), which is the method being proposed for the Ecovillage of Loudoun County (VA). Since Hartland Farms has agricultural activity, this seems like a good choice. Another option is to combine CLF with graywater, and deliver the mixture to the root zone of non-edible plants. This combining of CLF and graywater is controversial even within the composting toilet community, yet I believe it makes good sense. Graywater itself has low nutrient value, so it is enhanced by the high nitrogen CLF. However, most schemes for treating graywater by delivering it to the root zone of plants have been done in climates warmer than VT - it is likely that a greenhouse would be needed in Hartland to make this type of system work. My feeling about this is that the expense of a greenhouse only makes sense if the CLF and graywater are combined, and if there is a person(s) who wants to manage the greenhouse year-round as a business or hobby, producing non-edible crops of value.
Graywater is not entirely innocuous - it has biochemical oxygen demand (BOD), which is an indication of organic matter present, based on the demand for oxygen necessary to decompose it. Putting a high BOD waste stream into surface water can de-oxygenate the water, upsetting the ecological balance necessary for fish and other aquatic organisms. When directed to a leach field, it can cause an anaerobic biomat, leading to premature system failure. Graywater also has total suspended solids (TSS), which also can clog leach fields. Care on the part of the users can greatly affect what is in graywater - careful choosing of soaps and avoiding putting food waste down the drain (no garbage disposals) produce a more benign graywater, and one with fewer nutrients. This should be easy to achieve in this community.
My suggestion for nutrient recycling is as follows: use composting toilets and apply the compost to non-edible crops such as fruit trees; drain CLF by gravity to a central tank at the bottom of the site, where it can be transferred seasonally to a vehicle which can apply it to agricultural land; and drain graywater by gravity to a down-sized conventional septic system. Note that these are two separate drain systems. An alternate to the CLF drain system is to collect the CLF in a 300 gallon tank in each house (perhaps a larger shared tank in a duplex) and remove it periodically for land application, as is proposed at the VA Ecovillage. This has the disadvantage of requiring a pump to move the CLF from the bottom of the composter to the tank, and probably another pump (or the same pump and some isolation valves) to move the CLF to the vehicle which does the land application. I would rather pay for pipes than pumps!
If there is a clear desire for a greenhouse, combine CLF and graywater and use in the greenhouse. This requires some filtration equipment and a sizable greenhouse (depends on graywater flows - likely to be 2000 ft2 or more).
Graywater should be held to a minimum by the use of: low flow showerheads and faucets, horizontal axis clothes washers, low water use dishwashers (if any dishwashers), and no garbage disposals. The regulators should be persuaded that graywater produced will be reduced by at least 50% over typical housing, due to the waterless toilets and these measures. Heat should be recovered from the graywater before it leaves the houses. The community may want to offer to the regulators that they will measure graywater production, and also test the CLF for fecal coliform before a land application event.
Some people advocate urine-separating toilets (available from Scandinavia) and collecting the urine separately. This greatly reduces the chance of pathogens in the liquid. The problem I see is that CLF has been treated by the composter and doesn't smell, whereas raw urine smells bad quickly.
On using composters: commercial composters cost about $4000 each (!- made in small quantities). One potential advantage of a duplex is that both units might share one composter. Here are the hitches: the bathrooms must be back-to-back; and the residential composters are really only set up to accept two dry toilet chutes. Housing units with more than one toilet in a duplex sharing a composter would need to use a micro-flush toilet (such as the 1 pint Sea Land) for the second toilet. This will generate substantially more water use and CLF production. An option to consider: design the houses such that the toilet is in a separate compartment, so that occupants can be using the toilet, the shower, and the lavatory sink simultaneously. In a second home I designed on Cape Cod, which gets a high occupancy, three of the bedrooms share one bathroom. Entering that bathroom, one is in a room about 6' wide by 5' deep, which contains two lavatory sinks. Off of that room on one side is a small toilet room, and off it on the other side is a tub/shower room. All three functions can be used at the same time by as many as four people. This replaces two bathrooms.
Composters need to be kept warm (>65F). If in unheated basements, some small amount of heat may need to be added, and/or they may need to be built-in to a small insulated room.
I am inexperienced in community water supply systems, so here are thoughts which need to be tested with the hydrologist and the civil engineer.
Criteria include: cost, minimum energy, minimum number of moving parts equipment, minimize contact with potentially harmful materials (some plastics), design in potential for electricity-off usefulness (using gravity).
I think that the community should work hard to get the State to accept realistic water use numbers. With composters and the low flow showerheads and faucets, horizontal axis clothes washers, low water use dishwashers (if any dishwashers), and no garbage disposals, the usage should be no more than 50% of normal levels. The house in Hanover shown in EBN has been using an average of 55 gallons/day, with flush toilets and vertical axis washer, and two adult residents. Water use should be metered so the data is accumulated. Persuade the State to accept a reduced system with the offer to increase capacity if water usage proves to be greater than assumed.
Potential strategies:
If this wouldn't cause a regulatory battle, it would be worth considering whether a well is needed at all - my NH colleague Doug Clayton has a rainwater harvesting system and a 6200 gallon cistern and no well. Rainwater harvesting is common in other parts of the US (I have a design manual from TX). Water is filtered depending on the nature of the use. I'm assuming that this is beyond the pale of possibility in VT, but I throw out the idea. It's a low energy water system, since the water doesn't need to be pumped up from a deep well, and it avoids the disturbance of drilling a well way up on the hillside.
For the conventional solution - is it possible to have the system work similarly to a single house, in that there is a pressure tank in each building, which is pressurized by the well pump? It seems that if the well pump only fills a storage tank, then a second pump(s) will be needed to pressurize the house systems. There is probably a way to incorporate a non-pressurized storage tank for emergency use which can be kept topped up but normally valved out of the system. I will look at the proposed system when I return in early September, and work with the system design engineers to try to accomplish these things if it makes sense.
Collect rainwater run-off to use as irrigation water for the area around the houses and the gardens. Use this to further reduce well and storage tank size - offer to not use well water for exterior uses. Gutters work badly in VT - they ice up and fall off! Instead might we design in good drainage around each building, and connect the footing drains together and drain them to a central holding tank, from which water can be pumped for irrigation. (This is a possible use for some of the old concrete silo - re-assemble as a below grade tank structure, possibly lined, to store the run-off water. The top of the tank can double as a floor for a patio, gazebo, outbuilding, farm stand, etc., saving the cost of an additional foundation. This is potentially applicable to the biogas digester, too.)
The current base case house we are projecting has better-than-average insulation (7" strapped wall - nominal R-28), airtight construction, probably Canadian fiberglass "superwindows", no heat recovery ventilation. We haven't gotten to the level of detail of deciding about foundation insulation, but the cost estimate is low in this area right now.
The base case mechanical system assumes a Tarm wood gasifier boiler for each pod of 6-8 houses. The largest model is rated at 198,000 BTU/hour. The design load of a 1200 ft2 unit with insulated, heated basement (estimate, based on 20x30 2 story unit) is about 18,000 BTU/hour, so 8 units are about 160,000 BTU/hour. The output of the boiler may be more taxed by domestic hot water (DHW) loads than by heat (design work needs to be done to assess this) but water conservation measures and the possibility of heat recovery on graywater will help. Another variable is amount of DHW storage. I suggest that DHW make-up is prioritized in the control sequence over heat (houses like this only lose a degree or so per hour if unheated). The system may include a central storage tank, to increase capacity. Boiler water is circulated to each unit for heating (and possibly DHW - see below.) Heating distribution is possibly European-style steel radiators - good zoning, less clunky than baseboard fintubing, not too expensive - probably piped with polyethylene PEX tubing. Details need to worked out once we have a site plan and house plans. I expect a mechanical engineer will be involved, working with me.
It makes sense to me to back-up the wood boiler with a separate propane boiler. Propane is cleaner than oil, and can be vented without a chimney flue. There is also more risk of fuel spills with fuel oil. Propane costs more per unit of energy, currently as much as double fuel oil (oil has 50% more fuel value/gallon, and costs less).
The propane boiler has these functions: it provides small amounts of heat in the swing seasons when the houses want just a little heat and no one may want to fire the wood boiler. It provides DHW whenever the wood boiler is not being fired. Even if there is solar DHW (SDHW), there will be some back-up needed. Having the propane boiler also provides some redundancy if there is a failure of the wood boiler, and provides extra capacity for peak loading in extraordinary conditions (lots of people visiting at Christmas and the temperature drops to 30F below.) It provides a margin of safety that will allow more units per pod.
Those who choose to not use any fossil fuels may choose to skip the propane boiler, install graywater heat recovery, a storage tank, and perhaps SDHW, and fire the wood boiler whenever needed. It would be best if an entire pod made this decision!
DHW can be designed in two ways. The first is like most houses - each dwelling (or maybe each building - this depends on whether you choose to meter thermal energy) would have a DHW tank which has a heat exchanger that uses boiler water to make DHW. So boiler water comes into a unit and makes either heat or DHW or both. If there is a laundry in the same building as the boiler (likely), then there needs to be a tank for the laundry, too, and you need to decide how to bill for this energy (although most wash can be done with cold water).
The second approach is to place a central DHW tank next to the boiler, and circulate DHW through a separate piping loop. This necessitates a small pump so that there is always hot water in the loop, ready to serve a user without a long wait. It is also more piping. The trade-offs have to do with trading 6-8 tanks (or 3-4 larger tanks if shared) for one large one, and vs. the pump and recirculation loop with its electrical usage and higher thermal losses (at least the thermal losses all end up in the basements, since the buildings are linked with pipe which vents to the basements). One potential advantage of this approach occurs if there is SDHW - it will be easier to heat or provide preheat to one central tank than several distributed tanks, the solar array is centralized, probably smaller, and therefore cheaper to buy and install, and you can take advantage of diversity of multiple loads (when you are on vacation with individual SDHW, your neighbors can't use the output of your system in their house.) The laundry is served by the central tank, too. The central system makes it more difficult to do individual SDHW, although not impossible.
Whatever option is chosen (and it can vary by pod), piping to support future SDHW should be included, and the systems should be designed to accommodate graywater heat recovery and SDHW if they aren't installed initially.
It is always risky to make predictions about energy use, especially when nothing has been designed yet! And much depends on user behavior, which is why metering is good, at least to give feedback. At Pine Street Coho, there are two identical units, all energy is electric (ground source heat pumps), so it is easy to meter. One unit is using roughly double the electricity of the other! Also, especially when you get to very low energy use buildings, the annual variation in the weather, both solar availability and how cold it is, causes significant variation from year to year. The superinsulated active solar house in Hanover has varied almost two to one in annual heat/DHW energy use in four years. With these caveats, here are some estimated loads and energy usages (hand calcs, no computer modeling):
Based on the envelope assumptions above, and a 1200 ft2 unit (this is currently the size of a 3 BR 2 B), I estimate a heating load of about 24 MMBTU and a DHW load of about 10 MMBTU (41 gallons/day). This is 1.5 cords of wood for heating (based on 21 MMBTU/cord, 75% efficiency) and 0.6 cords for DHW. I assume that 10% of the heating load will be carried by propane (91,600 BTU/gallon, 84% efficiency, can likely do a bit better - 205 gallons of propane equal to 1 cord of hardwood). I assume, without SDHW, 50% of the DHW load will be carried by propane. So annual usage is about 1.65 cords of hardwood for heat and DHW, and 100 gallons of propane. For wood at $95/cord, and propane at $1.00/gallon (bulk purchase will help), this is $158 for wood and $100 for propane, total of $258/year. The cost of wood is half the cost of propane, per unit of energy delivered.
6 houses therefore use 10 cords and 600 gallons of propane, 8 houses use 13 cords and 800 gallons of propane. Provision must be made on the site plan to deliver and store these fuels.
If graywater heat recovery if implemented (assume $400/unit, may gang duplexes on one recovery device?), assume 1/3 of the annual DHW energy is recovered - save 0.1 cord and 22 gallons of propane - cost savings are $31/year. If SHDW is implemented, assume 2/3 of the annual DHW energy is recovered - (assume $3000 per unit), save 0.1 cords and 52 gallons of propane - cost savings $69/year. (SDHW produces more in the summer, so more propane would be displaced.) I should also add that if we install a large insulated water storage tank next to the boiler, it should be possible to get multi-day storage of DHW, so it is possible that the boiler would only need firing in the summer once every couple of days.
What do some of the envelope upgrades save? Going to foot-thick superinsulated walls saves the equivalent of about 0.2 cord in this small house. Going to heat recovery ventilation saves just under 0.25 cord. You can see, once you make a pretty good envelope, and you burn a low-cost fuel, that the cost/benefit ratio is not too attractive! However, these small savings are critical to a solar-heated house, in my opinion, because my goal in such houses is to spend as close to no money as possible on back-up heat (we used electric back-up in Hanover solar house.)
Which brings us to...
Through Dana, Amory and I have traded thoughts about the possibility of building houses that get all of their heat from the sun, passively, and require no heating system at all. Amory says its possible in this climate, I don't agree, and I did some computer modeling to test this. (Dana - would you write up a summary of the back-and-forths for those interested?)
In a nutshell - Amory touts truly state-of-the-art glass, R-11 or 12, mostly fixed winodows (not openable), with fabric-covered foam insulation strips around the edges to make the edges perform well thermally. I used glazing with less insulating value in my model, but with substantially higher solar heat transmission. Therefore, the same gain with probably less heat loss (the R-12 stuff doesn't let much sun in), and much less expense - less area, lower cost/ft2. Amory says the R-12 glass is a net gain over the heating season even on the north side of the house, even in VT. This may be true, but it is not relevant to making a 100% passive solar house. What makes a difference is what happens during the coldest, cloudiest part of the winter, when that glass is not gaining. In the last four winters we have seen two years with at least a week when the sun didn't shine - this is where the 100% solar idea fails. You can't store enough energy to ride it through, passively (and even with active storage, it costs too much.)
However, the kicker is that it is really difficult to go 100% solar on DHW, because it has a higher service temperature than space heating, so the energy is collected at a lower efficiency. So you need a back-up for DHW. Once you have that back-up, it makes sense to use it for heat. I just don't see a difference between heat and DHW - both are thermal loads, and if you can't do both at 100%, why work so hard to do just one? Once the wood boiler is there, the savings from the heroic measures aren't worth much. And Amory is suggesting using a propane water heater for the little back-up heat needed, plus DHW, whereas my inclination is, if propane is the back-up fuel rather than wood, to use one ultra-high efficiency propane boiler and pipe the heat around instead of six medium efficiency water heaters.
Finally - the European examples of ultra-low heating energy homes Amory cites are such that I think most of us wouldn't want to live in them. Very small glass areas relative to American homes. Because we have more sun here in the USA, we get to use more glass, but to get near the performance Amory claims, everything must be secondary to the energy issues. Houses are not just machines, there are many other important elements. It's just not a reasonable trade-off, I believe.
If some folks wanted to go further ecologically with their homes, here are some options. If you went down the Amory route, the houses would get more insulation (mostly in fatter walls), heat recovery ventilation (HRV) (Amory advocates bringing the fresh air to the HRV through ducts buried in the earth (earth tubes), which I've stayed away from due to possibility of mold), more glass and added thermal mass. No solar shading allowed. This is the superinsulated passive solar model. I estimate the back-up heat requirement for this house at about 0.5 cord of wood annually, or about 100 gallons of propane. Six of these homes together would only use about 3 cords of wood for heat. Adding in one third of the DHW load (assume the rest is carried by SDHW) yields a total of about 4-5 cords/year. This is a pretty small load to use a wood boiler on! The big storage tank would almost certainly be required, because the heat load would be so intermittent, and with SDHW, the DHW load also would not be constant. I'd be tempted, once the wood boiler is there with a storage tank, to skip the SDHW and fire the boiler in the summer. Total wood use would jump to 6-7 cords per 6 house pod, about 9 cords in a 8 house pod (all estimates based on the 1200 ft2 house size.)
The other advanced house option is superinsulated with active solar. With a whole pod of these, we would probably have collectors on the roofs of 1/2 - 2/3 of the houses in the pod, and I think one really large central storage tank, cast in as part of the basement of the central house. Something with an inside dimension of 18' x 15' would store 12,000 gallons, which is in the ballpark. Heating distribution would best be done by radiant floor, to allow use of low temperature water, so the heating system would also be more costly than that in the base case homes. To keep cost down, I'd suggest we look into creating radiant floors of acid-etched concrete - I've seen beautiful jobs done with this. Masonry floors will allow the lowest water temperatures for heating, so they make the solar system more efficient. Alternatively, I refer to the scheme mentioned at the end of the second systems memo, using massive interior walls as the heating distribution/storage combination.
Advanced home owners might choose photovoltaic (PV) to generate electricity. I have previously suggested connecting these systems to the utility grid, to eliminate battery storage and to share any surplus generated. We would look hard at what will be on the market when ready to build, because the technology is still evolving. Building-integrated PV (BIPV) is the way to go, because the PV replaces some of the weather skin of the building. PV cogeneration is coming to market, in which a single panel generates both electricity and heat.
Advanced homes might invest more money in low electrical use appliances and lighting. All homes would optimally have refrigeration on the exterior walls, and some might take advantage of this to build the refrigerator into the wall, such that it can be passively cooled by outdoor air in the colder seasons.
Advanced homes might incorporate more costly materials and finishes which have ecological and/or aesthetic benefits - natural materials such as stone, wood, straw, clay, which may cost more or less but often cost more to install.
We have discussed the goal of the Common House having some disaster-proof design features. These could include: rainwater harvesting and cistern storage, with filtration, for emergency water usage; grid-connected BIPV with battery storage, so that there is emergency power to run critical loads; super-efficient appliances, so as to minimize the battery storage to run these loads; SDHW, possibly PV cogen; graywater heat recovery; efficient wood-burning masonry heater or fireplace that requires no electricity, and possibly heats water, too.
Other eco-upgrades might include: superinsulation; enthalpic heat recovery ventilation with earth tube preheat for the coldest weather (CH is a good place to try the earth tubes); passive cooling for the refrigeration, especially if it includes a walk-in cooler; root cellar; interior planters to treat graywater - this is more a demonstration system, because there will be a central graywater treatment system, but it will be very beautiful; natural materials and finishes upgrade.
One possibility is that wood-fired cogeneration will reach the market in reasonably-sized and priced units. It would be good to allow for replacement of the wood boilers eventually with something like this, and/or to allow for future linking up of all the buildings without major disruption in case the right scale of the product is community-wide rather than pod-wide.
Space should be allocated for more and more PV as time goes on, so wiring chases and unobstructed roof areas should be designed in. Piping for future SDHW should be built-in.
The possibility of a methane producing digester should be considered. This would be located near the barn. Note these constraints: to make biogas, you need to collect manure. Pasturing animals means that some of this resource is impractical to collect (I assume?), so it is only feasible in the colder parts of the year. When it is cold, some of the biogas will be burned to keep the digester warm, since biological activity stops when it gets cold. When pressed to tell us how many cows made economic sense to invest in a digester to produce gas which is burned to cogenerate electricity, the VT DPS folks Dana and I met with told us 300 cows. This is not feasible on this farm. I think the best use of the gas is to serve thermal loads, either agriculture-related (heat for the cheese-making, etc.) or possibly for cooking. My quick calculations showed me that collecting the manure from one cow and digesting it, not counting the gas needed for the digester heat, might provide cooking fuel for one house. This would need to be verified. (BTW - one reference I have equates one cow to 24 humans for biogas production, so if the community has the equivalent of 50 adults, that is only two cows!) Digesters work best with the proper carbon/nitrogen ratio, so plant matter as well as manure should be added to the mix. I have some reading material on digesters if someone wants to do some research.
- Marc
Rosenbaum
|
|
|
|
|
|
|
|
|