Sunday, January 5, 2014

Hello neighbors postcards!

image-128

Some of my new neighbors might be curious about the activity on their street, so I made up this postcard that Ill drop in their mailboxes. I thought it would be nice to let them know whats going on, but really Im interested in getting started on my ambitious plan of world domination — by convincing everyone to live in ultra-efficient tiny houses!

Here are some of the most striking energy efficiency features of the Potwine Passive House, as listed in the postcard. Ill talk more in depth about these in future posts.

What is a passive house?

Originally popularized in the seventies by back-to-the-earth types, the passive house concept didn’t hit the mainstream until the early nineties when Professors Fiest (Lund University, Sweden) and Adamson (Institute for Housing and the Environment, Germany) took up the concept in earnest using a strictly scientific approach. The central thesis is that the greenhouse effect can be exploited in buildings to provide most of their heating needs. So much heat is available from the sun, in fact, that the original idea involved generating all of a building’s heat from the sun — hence the name passive: no active or mechanical processes would be required. A building would simply take care of its own temperature inherently, reminiscent of the clever designs of ancient buildings that functioned astonishingly well without a modern furnace or air conditioning. Today’s passive homes use many active mechanical systems like air conditioning and air ventilators, but the majority of heating comes passively, from the sun.

South facing windows are the critical source of heat

img-alternative-text

South facing windows can capture a tremendous amount of the suns energy in the winter while the sun is at a low angle in the sky. In the summer, when the sun is at a high angle in the sky, an overhang blocks light from hitting the windows to prevent excess heating. Windows facing west, east and north are kept to a minimum, if possible — they will lose heat while adding little solar heat gain. The challenge is to keep the south facing part of the building livable: too many windows will generate too much heat during the day and lose too much heat at night. We want just the right amount of south facing windows to generate enough heat, but no more.

R6 insulated triple pane windows make a huge difference

img-alternative-text

Windows are terrible insulators; they incur the greatest heat loss in most buildings. Increasing the insulating capability of the windows will result in enormous energy savings.

The actual amount of heat loss expected from a given type of window is quantified by a metric called the U-value, a measure of how much heat is lost per temperature difference between the inside and outside. A good double pane vinyl window will have a U-value of 0.37, given in the (stupid) US units of [Btu/hr SF ℉]. We’ve elected to go with German Kneer-Südfenster triple pane UPVC windows with an amazingly low U-value of 0.167, lower by more than half the U-value of a typical double pane window! This means that our triple pane windows will be retaining more than twice as much heat compared to double pane windows, and this will make a huge difference in energy savings.

The U-value is the inverse of the R-value, the common metric for insulation. Converting the U-value to the R-value of our windows gives an R-value of R6.

Concrete thermal mass foundation stores heat for the night

img-alternative-text

What happens at night when the sun goes down and it is cold outside? We want to be able to store the heat gained during the day and use it at night. That is, amazingly, exactly what the concrete slab foundation will do! The foundation of the home consists of a concrete slab sitting on top of a layer of insulating foam. The concrete serves as a huge reservoir for heat, helping to keep the temperature constant inside the home by absorbing heat during the day when the sun is out, and by releasing heat at night when the temperature inside cools down. The slab is insulated from the ground to prevent stored heat from escaping through the earth. The trick is to figure out how much concrete is needed to absorb a sufficient amount of energy during the day to keep the home warm at night. Too much concrete will never allow the home to get warm. Too little concrete will never be able to keep the home warm all night. The architect uses an energy model to decide how thick to make the concrete slab.

R50 insulated prefabricated wall panels

img-alternative-text

The walls are put together in a new and interesting way that helps lower cost, save time and reduce air leaks. Huge wall panels are prefabricated ahead of time in a factory setting, shipped to the building site and then hung on the frame. The installation process only takes a couple of days. The huge size of the panels minimizes the little cracks and seams that cause air leakage in traditional walls. The panels are called Structurally Insulated Panels (SIPs). The SIPs are 8" thick. A normal 2"x6" wall is added to the interior of the SIPs and filled with blown-in fiberglass, resulting in a super insulated wall assembly with an astronomical R-value of R50.

Heat recovery ventilation keeps the air fresh

A typical home leaks a lot of air to the outside. It’s equivalent to leaving the front door completely wide open! In many cases, the leaks are there by design: they bring in fresh air. But when a home is tightly sealed, things will get stinky pretty quickly unless you have a way of ventilating the place. The problem is that you can’t just open a window because you’ll lose heat (or cold) and defeat the whole purpose of tightly sealing the home. The solution is absolutely ingenious: a Heat Recovery Ventilator (HRV). The HRV brings fresh air into the home and simultaneously exhausts stale air to the outside, yet miraculously doesn’t allow heat to escape. I’ll explain how this works in a future post; it’s very clever.

Drain water heat recovery saves hot water

Hot water is a tough issue to solve. Heating up water requires a tremendous amount of energy. Many choose to employ natural gas water heaters, which are probably the most economical option (at least until the natural gas bubble bursts). Our goal is to go fossil fuel free, so we had to consider other options. In a future post, I’ll talk about why we ended up choosing on-demand tankless water heaters instead of solar thermal hot water or a heat pump water heater.

img-alternative-text

For the moment, however, let me tell you about one of the really cool aspects of our hot water system. Think about what happens when you take a shower. Hot water pours over your body and disappears down the drain. All that energy goes into heating the hot water, but most of the energy is immediately waisted down the drain. Enter the Drain Water Heat Recovery (DWHR) pipe: a drain pipe that recovers most of the heat in the water flowing down the drain and magically transfers it back to the inlet of the water heater. The operating principle is similar to that of the HRV.

LED lighting saves electricity

img-alternative-text

LED lighting represents a dramatic improvement in lighting technology, providing higher efficiency than incandescents, better color than fluorescents, dimmability and cost savings over the ridiculously long 40 year life of the bulb. Like most energy efficient options, the upfront costs are high, but you save money in the long run. We are outfitting the house with PAR30 (short neck) LED track lights. These bulbs are larger than your standard GU10 or MR16 halogen replacement track light bulbs, but they provide more light, are cheaper and are more efficient. A 12 Watt PAR30 LED replaces a 75 Watt halogen bulb, saving an amazing 84%!

Other cool features

Check back here to find out more about the other fascinating energy saving features of the Potwine Passive House, including:

  • An efficient box-shape building structure that retains heat extraordinarily well
  • A bunch of clever space saving architectural elements that make a small home appear more spacious
  • A 4kW solar photovoltaic (PV) array that will provide 100% of energy
  • Tankless on-demand water heaters from Stiebel Eltron, providing instantaneous hot water that never runs out, all in a tiny package at a low cost
  • An efficient Fujitsu heat pump, providing heating or cooling in one unit, at an efficiency more than two times that of a conventional heating system
  • An efficient heat pump dryer, more than twice as efficient as a conventional dryer
  • An efficient induction stovetop that boils water faster (while using less energy) than any other stovetop
  • An efficient steam oven that will cook a whole chicken in 20 min!
  • A recirculating range hood from Vent-a-hood
  • An eMonitor energy monitor that tracks the electricity usage of every circuit in the home

It’s not just a bunch of cool gizmos

Overall, I want to point out that the home is not just a bunch of really cool technological gizmos. It’s a fundamentally new approach to assembling a building. Beginning with the concept of using solar heat, most of the other features derive from this simple first step: a lot of insulation and a tight building envelope to hold in the heat, an HRV to bring in fresh air without losing heat, and a concrete slab to store the heat for later. The second important aspect is the elimination of fossil fuel combustion in favor of electricity. It just so happens that things that use electricity are way more efficient than things that burn fossil fuels. We benefit from this increased efficiency when we install the PV system, which will be much smaller than it would have been without all the super efficient components listed above. In a nutshell, it’s a smarter, more holistic approach. That’s how we expect to achieve a home that uses 8 times less energy than the typical Massachusetts home.

5 comments:

  1. Alexi, are there links to pictures of homes elsewhere already using these technologies? This is super cool stuff!

    ReplyDelete
  2. Yes! Here's a good article with pictures of some amazing passive houses:

    http://www.houzz.com/ideabooks/21075737/list/The-Passive-House--What-It-Is-and-Why-You-Should-Care

    ReplyDelete
  3. Hi Alexi,

    Vivian told me about your house project. Looks great!

    Why not use an energy recovery ventilator instead of a heat recovery ventilator to also maintain relative humidity in 40-50% range? It's been really dry this winter and the past couple of summers have been pretty humid.

    ReplyDelete
  4. Great question Matt!

    We thought long and hard about the choice between and a heat recovery ventilator (HRV) and an energy recovery ventilator (ERV). Both provide fresh air to the home while recovering heat from the vented air, but only the ERV captures moisture as well.

    There are two problems, however, with the ERV: it's less efficient than the HRV and the moisture recovery doesn't work well in the winter.

    Since we might be close to the maximum heat load allowed by the passive house standard (15 kWh/m2 per year), it would have been risky to go with the ERV, which would have increased the heat load slightly.

    The ERV moisture recovery feature works by drawing moisture across a semipermeable membrane. In order to function properly, there must be a large difference between the moisture content of the air indoors and the air outdoors. In the summer, everything works great because you'll have a moisture content of approximately 30 gm/m3 outside and a moisture content of approximately 10 gm/m3 indoors, giving a large differential of 20 gm/m3. But in the winter, on a typical day you'll have 2 gm/m3 outside and 4 gm/m3 inside, resulting in only a 2 gm/m3 difference. Warmer air can hold a lot more moisture than cold air. The small moisture differential in winter is apparently not sufficient for the ERV to capture moisture effectively.

    I got the above numbers from the hyper physics website:

    http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/relhum.html

    In the end, the HRV is the better option because it's more efficient, yet won't really result in any difference in humidity in the winter compared to the ERV. Since the heat pump will remove moisture in the summer, the ERV is somewhat redundant. It's true that the ERV will save energy in the summer because the heat pump won't have to work as hard to remove moisture, but if you look at how much energy the heat pump will use in the summer: it's tiny (see my early post about energy usage).

    ReplyDelete
  5. Wow.. Nice blog. Print Ultra thick postcards from PrintMagic you will find two types of thickness and two types of coating. 28pt and 32pt are our thickness best options. Consider your desire for durability and bendability when choosing between these two best options. 32pt thick postcards and will be the best for your need for durability. Keep in mind this thick postcard printing and does not allow for any bend. We also offer Matte our uncovered best option to allow for a more raw and exposed look. Try our semi gloss best option for a more glossy shine to your ultra thick postcard’s surface. This is just the beginning of your best options. Enjoy browsing through our many paper stocks to find the exact look that you need for your next ultra thick postcard printing.

    ReplyDelete