2019-02-08: I really enjoyed the presentations last night. Overall, I think the technical quality of your work is stronger than is sometimes found in APSC 100 projects, and that’s the main goal of this pilot with a more involved faculty advisor. Here are some general comments to help you keep on with that high performance.

A lot of this stuff is already covered in other parts of the book… and I would also like to add a caution about weighted evaluation matrices. All of the numbers in those tables are subjective assessments made by your group and don’t necessarily have the advantage of long experience. When you total those numbers the scores may seem more real because they are quantitative scores, but remember the subjective sources. Critically assess your results to be sure they match up with what you expect. If I see outhouse, composting toilet, and low flow flush toilet getting similar scores, I will really want to see a paragraph of analysis. Maybe low initial cost is a really crucial factor, but as a habituated potential user, I like flush toilets.

Green, Off Grid Heating Solutions

Although the client specified “off-grid,” I think a big part of that objective was reduced carbon footprint. Only about 10% of Ontario’s on-grid electricity is produced from fossil carbon, so your target operation should do better, unless you are in a location where the grid is unavailable. In a remote location the comparison might be diesel generation, 100% fossil carbon sourced and easier to improve on.

Propane

Many of you suggested propane as a heating fuel, either primary or backup. Propane is a great solution for heating and cooking anywhere without a natural gas supply, however both are fossil fuels with significant carbon footprint. Your final analysis should show that only small amounts are needed over the course of the year. The same applies to gasoline for your backup generator.

Wood

Montreal has banned most wood burning due to emissions issues, so be sure you address that if you plan on wood heat. Also keep in mind that wood heating is only carbon neutral if you regrow as much wood as you burn.

Heat Pumps

There was surprisingly little mention of heat pumps in your design plans. This surprised me as they can run on the green electricity from your solar panels and even at -25C they will add 2 kWh of heat to your house for every 1 kWh of electricity you use to run them. That coefficient of performance will go up to 4 or more if the outdoor temperature is more moderate, or if you use a warmer heat reservoir with lake water or a ground-source heat pump, sometimes referred to as geothermal. I’m expecting heat pump systems of some kind to show up as the primary heating source in most of your designs unless you make a strong case for something else.

Solar Energy

It’s hard to get greener than solar. Even our tiny house footprint provides enough area for significant solar collection, thermal or photovoltaic.

Passive Solar

Several groups mentioned design for passive solar gain in the winter. All groups should be paying attention to them.

Solar Thermal

Every location you use for a solar thermal collection panel is area you can’t use for passive solar from windows or PV solar to charge batteries, so you will need to think about the tradeoff on your limited footprint.

Solar thermal is challenging in Canada’s cold winter, but can be combined with heat pumps for more effective joint water heating systems. Here’s an example from Queen’s.

Solar PV and Aiming

Some of you talked about adjusting your solar panels to maximize your total output. Typical values for Kingston show about 40% increase in total annual output for continuous, two axis mechanical tracking of the panels to follow the sun. Note that the biggest gains come in high summer when you may have excess energy even without tracking. That excess energy is only valuable if you can put it back into the grid. Active tracking has become much less common as the price of panels has dropped while the price of mechanical tracking systems has increased. Ask yourselves if this is the right technology for your tiny house.

There are also output advantages to be had by adjusting only the elevation angle of a south facing panel, more vertical in the winter and more horizontal in the summer. I can see advantages to both fixed panels and manual adjustment twice a year to match the seasons. You will need to pick angles that best match your needs at different times of the year.

Water and Sewer

There are good reasons that a well and a septic tank/tile bed are a common solution when you are off municipal water. I’m not surprised to see that solution advanced in rural locations. Check bylaws to see what’s allowable inside the city.

Consider the advantages of split systems. Water from lakes, wells, and collected water should be treated for drinking, but is probably fine for showers, etc. without treatment. How much water is required for actual drinking and cooking?

If you are considering water capture and storage, be sure to demonstrate the actual advantages you achieve in a Kingston context. There are a lot of places in the world with serious water shortages, but Kingston isn’t one of them.

Energy Storage

Energy storage is going to be critical to your success and needs some more attention. Most of you either didn’t mention it or just mentioned lithium ion batteries in passing. You will need to give serious consideration to both type of battery and to the capacity you need to install.

Lithium Ion  are highly efficient, light weight, long life, and can be rapidly charged and discharged, all of which make them especially good for mobile applications from phones to cars in scale. They are also very expensive.

AGM Lead Acid are low maintenance, less efficient, moderately priced.

Flooded Lead Acid require regular maintenance of water levels, are a little lower performance than AGM, and are the cheapest.

All three are used in off-grid or power backup systems and you should evaluate the advantages and disadvantages. You will have a better handle on capacity requirements once you do your detailed calculations. The more storage you have, the higher the cost, but the less you have to worry about needing backup power on cloudy days.

Other factors

Look critically at your floorplans for actual usability. A common problem is not providing sufficient clearances to allow comfortable movement in the space. Once you get some details, start asking which sharp corners you will bump into and test for practicality with mockups.

Ventilation is going to be important in your well sealed tiny house. Consider ventilation systems with heat recovery air/air heat exchangers that will get you fresh air with minimal heat loss.

Energy and water budgets will be critical. As you move forward, identify specific products and actual consumption figures. You tiny house will have systems that are not typical of “average” power consumption or water use and the sooner you get solid estimates the better your plan will be.

Hydro electric power would be a great solution if you have a site that facilitates it with enough water flow and enough elevation change. The power will be

\[

P = \rho g H Q

\]

so you could get almost 10 kW from a flow of $1\units{m^3/s}$ dropping 1 meter at 100% conversion efficiency, but that’s a lot of water and the efficiency will be much less than 100% for low elevation changes. If you propose hydro for your location, I will want to see an actual location specified, with details of the available elevation and flow rate, and how it varies over the year.

My rough guess for energy

I’ll have 10 panels that would generate 3800 W in full sun, but I only get the equivalent about 3 full sun hours on a typical winter day, so 11.4 kWh can go in the batteries. If I use 4 kWh running stuff inside the house, all that energy will remain in the house as heat, and I will still have 7.4 kWh to run a heat pump that should bring me at least 15 kWh of heat into the house. I’ve ignored the inefficiencies of the storage system that will reduce the energy, but I’ve also ignored solar heat gain, so it should balance out.

I can heat my house on all electric solar on a typical day if I can keep the losses down to 19 kWh. (and I can only shower on days when I have excess energy to heat the water 😉 )

You should be working through estimates like this for your house as you improve the detail of your energy budgets with a target of burning as little propane as possible.