Passive Cooling in a Tiny House

Published

2 years ago

June 11, 2021

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Originally published on RMI.org.
By John Matson

As the world warms and the demand for cooling increases, many homes will require an “all of the above” approach to keep cool without further contributing to global warming. That can include high-performance cooling systems that use climate-friendly coolants and consume relatively little energy, as well as building design approaches that offset the need for mechanical cooling in the first place.

This tiny, ultra-efficient Passive House was built in just 10 days in #Australia http://t.co/glUiZVSN1o pic.twitter.com/pjYxo496ti

— The Architect’s Newspaper (@archpaper) February 10, 2015

In this post, we look at some passive cooling strategies that help keep an innovative tiny house comfortable during California summers, without the use of a mechanical cooling system.

Brett Webster, a manager in RMI’s Carbon-Free Buildings program, lives in a 170-square-foot home in Sonoma County, California. Brett and his partner helped design and build the solar-powered tiny house as part of a graduate project, and they have lived in the demonstration home for about five years. The home itself was built on a 24-foot-long trailer and can be hitched up to a truck for relocation. So even though Brett and his partner have lived in their tiny home for years, they have moved twice in that time between Northern California locations (and their respective microclimates).

Strategic Shading

The walls of the tiny house are clad in reclaimed cedar slats over one-inch-thick panels of cork, which provides a layer of continuous insulation, reducing the thermal bridging of the wooden wall framing. Because the carbon sequestered in cork trees can exceed the carbon emissions of producing cork products, cork is often considered a carbon-negative material. The cedar siding is separated from the cork by an air gap, which allows the wooden slats to shade the cork and absorb solar radiation, while slowing the rate of heat transfer directly to the house. The walls of the structure are insulated with recycled denim to further limit heat gain in warm weather and heat loss in cool weather.

Pulley-mounted shade awnings, made from cedar slats to match the siding, cover the largest expanse of glass on the tiny house: a sliding-glass door at the entry to the home. Webster says that the shade structure extends far enough to block solar radiation from pouring through the glass entryway in summer, but it can let in sunlight and heat in winter, when the sun is lower in the sky.

The ability to shade the windows in summer and admit sunlight during the winter is critical to maintaining passive comfort in the house. The windows that the design team chose for the tiny house are well-insulated (low U-value) but are also designed to let the sun’s heat in (high solar heat gain coefficient), because the Bay Area is mostly a heating-dominant climate zone. During the summer, when that heat gain is not desirable, shading the windows is a necessity.

Ceiling and Roof

A layer of BioPCM phase change material in the ceiling acts like thermal mass to absorb and store heat that would otherwise warm the interior space. Adobe buildings and concrete-walled structures similarly benefit from thermal mass that prevents the interior from becoming overheated during the day. But phase change material is lightweight, making it more appropriate for applications like the ceiling of a tiny house, and it doesn’t have the carbon footprint of concrete. (Cement production alone accounts for about 8 percent of global carbon emissions.)

The phase change material, which comes embedded in sheets that can be rolled out between ceiling joists like high-tech bubble wrap, melts from solid to liquid at 77 degrees F (25°C). As it changes phases, the material absorbs a lot of thermal energy, preventing the temperature from exceeding 77 degrees until its heat-absorbing capacity has been reached, like a sponge that can’t soak up any more water.

The tiny house’s roof is designed to harness much of the sun’s energy and reject the rest. A 2.3-kilowatt solar array shades much of the tiny house’s roof and feeds into a Tesla Powerwall to store electricity for nighttime use. The “cool roof” is also covered with a light-colored acrylic roofing membrane to minimize heat gain from solar radiation.

Some Energy Required (But Not Much)

In addition to the passive cooling approaches described above, the tiny house relies on a few efficient electric devices to provide airflow and ventilation. Even though they don’t qualify as strictly “passive” technologies, ceiling fans and other efficient electric devices have long gone hand-in-hand with passive cooling approaches. The ventilation and airflow systems in the tiny house consume very little energy and allow the building to remain comfortable without a dedicated mechanical cooling system.

A high-efficiency overhead ceiling fan consumes 4–18 watts of electricity and ensures occupant comfort in warmer temperatures. “Airflow creates a cooling sensation that’s extremely effective,” Webster says. According to the US Department of Energy, using a ceiling fan can significantly offset the need for air conditioning, allowing occupants to raise the thermostat by about 4 degrees F without sacrificing comfort.

The well-insulated structure is designed to be closed off to the outside during hot days in the summer, so the windows do not provide any natural ventilation during the daytime. The tiny house therefore relies on an energy recovery ventilator to bring fresh air into the house. An energy recovery ventilator uses a heat exchanger to reduce the thermal energy of the outside air before it enters the house, thereby providing ventilation without flushing warm air into the building. In the winter, it does the reverse, using the heat of the outgoing stale air to warm the incoming fresh air.

Unplugging

The tiny house’s passive design and minimal energy requirements for ventilation make it fully capable of going off-grid, especially in the summer months when solar energy is abundant. And even if most of us aren’t ready to commit to living in a 170-square-foot house on wheels, the lessons from Webster’s tiny house and other passive homes provide a powerful reminder: Even for energy-intensive applications like cooling, with thoughtful design, you can do a lot with a little.

Image gallery courtesy of RMI.

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Environment

Weird Alibaba: An inflatable Chinese electric jet ski for $2,000 – What’s the catch?

Published

9 hours ago

April 1, 2023

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Weird Alibaba: An inflatable Chinese electric jet ski for ,000 - What's the catch?

As usual for entries in this Awesomely Weird Alibaba Electric Vehicle of the Week column, the fun EVs we dredge up tend to bridge the gap between fun-looking and palm sweat-inducing. Would you take a cheap inflatable electric jet ski out into the bay or off the coast? What if I told you that you had to build it yourself?

That appears to be the case here with this week’s find. It’s an inflatable vessel that is jet ski shaped, though I’m not sure it fulfills all of the requirements to become a jet ski – namely the water jet turbine.

In fact, there’s actually no motor at all. It seems to be just the 3.5 meter (11 ft) boat itself, but at least it comes with a convenient transom in back to mount your own motor.

And in our case, we can slap on an electric outboard to turn this thing into not just a bad idea on water, but a green bad idea on water.

If you really wanted to stay true to the advertising, you could actually get an electrically powered jet ski-style water turbine to add to this boat. Amazon can hook you up with an impressive offering that looks like it would require cutting an inlet hole in the bottom of the boat and an exit somewhere through the rear transom.

Short of building a true jet ski though, I think an overpowered trolling motor will probably suffice. The vendor for the motor linked above seems to propose that is equivalent to 10 hp, which sounds reasonable for a small watercraft like this.

Technically the motor is only rated at 2.2 kW, which is around 3 hp. But we generally find that small electric outboards offer performance of around 3x the rated power of combustion engine outboards due to their much higher torque. It may not rip as fast as the larger gas engine below, but then again maybe it will. Who knows until we find out ourselves?!

You’ll need a whopping 60V of battery for that awesome little electric outboard, which I’m hoping will fit either under the seat or under the “hood” of the jet ski.

I’d actually be pretty interested to get a look under that hood to see what’s going on with that steering wheel. Since the jet ski/inflatable boat seems to be set up for a transom-mounted trolling motor, I don’t know how they expect to tie in steering linkage to something like that.

But my past experience of buying electric boats on Alibaba has taught me to never discount the ingenuity of East Asian engineers building low-cost vehicles that will presumably hold the life of one or more people in their hands.

One thing is for sure: At around $2k, this will definitely be the cheapest new jet ski you could buy, electric or otherwise. Personal watercraft aren’t cheap, and the electric ones carry a significant premium.

But if you’re handy, don’t mind wiring up a motor and battery yourself, and also don’t mind a steering wheel for show while you twist around to control a tiller motor, then you just might wind up with one of the most unique vessels on your local lake or river.

And consider the ease of transport! You probably don’t even need a trailer like you would for a traditional jet ski. The entire thing weighs just 176 kg (388 lb), though the spec sheet also says it is made from fiberglass, so perhaps the data isn’t quite accurate. Either way, this inflatable vessel can’t weigh too much. And the fact that you can deflate it to fit in the back of a van or SUV is a big benefit too. Or you can just leave it inflated and probably fit it in a truck with the tail gate down. Not my mini-truck, but maybe your truck.

At $2,025 for this thing, it’s pretty darn cheap – though that’s before the cost of batteries and a motor. Don’t forget though that there’d be several thousand dollars in shipping costs, customs import charges, taxes, broker fees, etc. Also, don’t forget that you should absolutely not buy this thing. While I’ve picked up some cool and weird little EVs from Alibaba over the years, it’s never a good idea. The process is long and complicated, not to mention fraught with extra charges at every step of the way. And you never know if the company who just received your wire transfer is even going to deliver your product in the end, which is just another fun little stressor that comes with shopping on Alibaba. So please, don’t join the ranks of my foolish readers and risk your hard earned money on something weird like this.

But if you ignore my warnings and decide to go for it, be sure to let me know what happened! And maybe update your will before the maiden voyage.

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Environment

Here are the best April Fool’s jokes from the e-bike industry this year

Published

11 hours ago

April 1, 2023

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Here are the best April Fool's jokes from the e-bike industry this year

April Fool’s Day, celebrated annually on the first of April, is usually a light-hearted and mischievous occasion marked by good-natured pranks, hoaxes, and jokes. Large organizations often take part in hoodwinking others, creating an atmosphere of amusement and (hopefully) harmless trickery. Sure, it’s annoying when you fall for it. But it’s also humorous to see what companies can come up with next. E-bike companies and the larger micromobility industry often have fun getting in touch with their inner prankster (remember the pedal-powered popemobile?!), and this year was no exception. These are some of the fun and light-hearted new announcements from around the electric biking and micromobility world.

We’ll keep updating as we find more, and feel free to send me any you find today (contact info in my author blurb below the article).

Magnum’s human-powered bike

Here in the e-bike industry we are often so deeply focused on the latest batteries or the most innovative new motors that we can sometimes forget our roots. Magnum Bikes, a popular electric bike company, wants to make sure we all remember where we came from with the “launch” of its new human-powered electric bike.

Called the Navigator Infinite, Magnum says the bike can get over 100 miles (160 km) of range. I guess “infinite” truly is at least 100!

Environment

Scientists have found major storage capacity in water-based batteries

Published

21 hours ago

March 31, 2023

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Scientists have found major storage capacity in water-based batteries

Texas A&M University scientists have been working with metal-free, water-based battery electrodes, and they’re finding that the difference in energy storage capacity is as much as 1,000%.

How the water-based batteries work

In the scientists’ paper, published in Nature Materials this week, the water-based, or aqueous, batteries consist of a cathode – the negatively charged electrode; an anode – the positively charged electrode; and an electrolyte, like traditional batteries. But in this water-based battery, the cathodes and anodes are polymers that can store energy, and the electrolyte is water mixed with organic salts.

The electrolyte transfers the ions – the charge-carrying particles – back and forth between the cathode and the anode, and the electrolyte is also key to energy storage through its interactions with the electrode.

Chemical engineering professor and co-author Dr. Jodie Lutkenhaus asserts:

If an electrode swells too much during cycling, then it can’t conduct electrons very well, and you lose all the performance.

I believe there is a 1,000% difference in energy storage capacity, depending on the electrolyte choice because of swelling effects.

According to their paper, the electrodes – the “redox-active non-conjugated radical polymers” – are promising candidates for water-based batteries because of the polymers’ high discharge voltage and fast redox kinetics.

However, the researchers note in their paper’s abstract:

[L]ittle is known regarding the energy storage mechanism of these polymers in an aqueous environment. The reaction itself is complex and difficult to resolve because of the simultaneous transfer of electrons, ions, and water molecules.

The future of aqueous batteries

The researchers suggest that water-based batteries might be able to mitigate potential shortages of metals such as cobalt and lithium, as well as eliminate the potential for battery fires.

Lutkenhaus continued:

There would be no battery fires anymore because it’s water-based.

In the future, if materials shortages are projected, the price of lithium-ion batteries will go way up. If we have this alternative battery, we can turn to this chemistry, where the supply is much more stable because we can manufacture them here in the United States and materials to make them are here.

The researchers also conducted computational simulation and analysis, and they’ll carry out further simulations to better understand the theory.

Chemistry assistant professor and co-author Dr. Daniel Tabor said:

With this new energy storage technology, this is a push forward to lithium-free batteries. We have a better molecular level picture of what makes some battery electrodes work better than others, and this gives us strong evidence of where to go forward in materials design.

Photo: Texas A&M Engineering

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