Viva Le Soleur

The days are short and time is flying. We move out of our rental by mid September. How exciting if not a bit trepiditious, what adventures will we face next. Sobering it is to think that this time last year we hadn't yet fathomed moving out of Christchurch, let alone be living in an eco village in Motueka. We eagerly await our next chapter, moving to our land and building our new life.

We have spent the better part of the last few weekends shifting stuff out from our garden shed back into our container. Seems like a pointless exercise, but I assure you there is a reason to our madness. Our landlord has decided to purchase a large garage which will be located in the position of our shed. Hence we had to relocate all the items in that shed to somewhere before we move up to our lot ... aha, the container! But as it so happens, we had already sold our container to a local dude. He was nice enough to let us have the use of it temporarily as we make the shuffle up to our lot, and then have it move sometime in early to mid November.

We were previously intending on building a smallish 30m2 sleepout to live in while the house was built. But we have since  realised that this is a rather expensive exercise, slow to get council consent, and will not buy us all that much storage space. So instead we will be building a weatherboard cladded pole shed (ie. a shed that is engineered as a post and beam construction). This seemed like a cost effective alternative as it buys us a 6m deep shed with two 4.5m bays - one of the bays will be unlined (the garage), and the other bay will be fully line and insulated to become an office and workshop area. Here is where I will carry out my crazy mad scientist like experiments. In the mean time however, this room could be fashioned into a make shift lounge and dining area.

As the pole shed isn't classified as suitable for human "habitation", we will initially be hiring a portable cabin to sleep in, and another one as my office. Additionally we will be hiring a portaloo and for the first month or so a porta shower too. Eventually we will be building an outdoor shower (with sufficient privacy of course) which will remain a permanent feature. I've always liked the idea of an outdoor shower where you can clean off after a grubby session in the garden - this would be especially useful for the mud child. There are many beautiful examples of outdoor showers, just do a google search to see what I mean :)

Der power haus

I was previously considering placing a solar aray on a structure of its own down the side of the hill, but have reconsidered this because:
- the mono slope roof of the pole shed is likely to be mostly north facing anyway, making it a good area to mount the panels on.
- the garage area is a great place to house the battery and the backup generator securely
- could also be a great place for interfacing to the grid if ever I wanted to sell excess power back to the power company.
- the compactness of this setup means that all the wiring will be much shorter and this lessens the losses in the system.
- easier to get access to the items that will require servicing (such as the genset).

I will still place the panels on a mountings where the angle of inclination can be adjusted, so I can optimize for the best angle. I even found a friendly local guy who has designed some. They are unfortunately manufactured in China, most probably from Aluminium smeltered in Tiwai Point, NZ? ... go figure eh! 

Recently I picked up my solar panels (all 14x 240W 24V panels), that sure took some effort to haul over from Nelson - those panels were heavy! The bloke who sold me the panels was nice enough to organise the panels to come up for free in a glass company truck which would otherwise have returned to Nelson empty up from Christchurch. That man deserves a DB! ... also the nice guy at the glass shop who help me carry the whole lot into the trailer, and the other nice guy at Atamai who helped me put them in storage.

I have also now purchased a 5kw diesel inverter generator which will be our backup power. I paid a bit more for the inverter function which means it can throttle down and vary the speed of the engine independently of the AC voltage out. This unit has a digital inverter output which regulates voltage and frequency to power the house directly. I'm hoping this equates to a more efficient light load performance. Running the generator hard to charge the batteries might be more efficient for the generator, but there is an efficiency penalty of going from mechanical->electrical->chemical->electrical

than just going

mechanical->electrical directly.

For the NiFe batteries that I'm getting, I may lose up to half of the electrical energy from the generator to heating up the batteries and generating hydrogen. However I will be monitoring efficiencies and seeing what works best, NiFe's may not start generating H2 until some voltage threshold for instance. The reason with going diesel is because the fuel is cheaper, and can be made multi fuel using a variety of vegetable oils and potentially biogas.

Passive Solar:

I've been reading up on passive solar design. This is the concept where you use sunshine coming into a window (called solar gain) to heat up a slab of thermal mass  (usually the concrete floor or wall), in a house that is well insulated. However there are variants of the passive solar design which I have been exploring.

Solar Slab design:

I've been reading Passive Solar House which is a somewhat technical book that discusses the use of a "solar slab". The solar slab is constructed with concrete elements with holes in it (like a pre-cast concrete block) aligned to the north-south axis. The floor is well insulated under this floor and on its sides (around ring foundations), creating an insulated thermal chamber that can store energy in the concrete elements. More concrete is poured over the top of this array of concrete blocks to a desired and calculated thickness to modulate the total thermal mass available for energy storage. The north and south end of the floor will have channels exposing the holes of the concrete blocks, and the top surface covered with flooring material fitted with gratings to allow air to flow.

Solar slab design

When the sun heats up the north side area of the house (seeing that we are in the southern hemisphere), the hot air will start to rise and pull the colder air from the south side of the house through the gratings and channels in the concrete blocks, and continues to circulation in a manner called a thermosiphon. It may or may not include the roof cavity in this circulation path, but if it did would benefit from additional heat gain. The longer this circuit works, the more the ambient temperature of the solar slab will rise - hence capturing thermal energy during the day time. At night the captured energy radiates back into the house via the concrete slab and flooring materials. If there is enough thermal mass designed into the solar slab, it will have the ability to thermally freewheel over the next days depending on how much of the heat in the house escapes. Because the solar slab absorbs thermal energy during the day time, this also prevents the house from overheating.  However a balance is required as too much thermal mass will require a cold house to take days to heat back up again. 

In winter when the sun doesn't shine as much and you don't get as much solar gain, a backup heating system is required. In our case we have chosen to add a wood cooker. This will be our primary means of cooking in winter but will double as a source of heat for our domestic hot water heating as well as ambient space heater. The book stipulates that the cooker's flue be piped into a hollow core wall which is also connected to the concrete blocks below (via a hole in the floor). This facilitates thermal currents in the concrete floor assisted by a silent electric fan located in the hollow core wall. In some cases ducts and fans may be also used to help distribute the heat better around the place - however the goal is to try and minimise the number of active elements required inthe system.Working out how to do this effectively is a bit of a challenge. The ideal aim is to store excess heat generated from the wood cooker into the slab using convective currents in the walls and floors - allowing mostly radiative heat to permeated into the living areas (no moving air).

Annualised Geo-Solar (AGS):

I happen to come across another idea which is this AGS system. Typically thermal freewheeling design is done in relation to a 24 hour period, or certainly only a few days at most. AGS attempts to stretch this much further, utilising an entire's summer of heating to cater for supplementing the house with heat throughout winter. This requires an enourmous amount of thermal mass and could take some time to come to thermal equilibrium (think years). The basic concept is simple enough, bury a large diameter pipe under the centre line of the house and pump (or thermosiphon) the hot air in the top of the roof cavity into this buried pipe. The depth at which the pipe is buried sets your thermal mass, which depends a bit on the thermal properties of the earth. Below this pipe you could insulate it with a strip of thick insulation and shiny foil to disrupt conduction and radiative thermal leakage to the ground below it. The thermal conductivity of the soil is poor enough that the heat you put into the ground appears at the top of the concrete slab roughly 6 months after it goes in.
AGS system

This requires that the concrete slab doesn't have insulation beneath it - the conducted heat from the ground needs to permeate the concrete floor to warm it up. However ring foundation insulation is necessary to stop it escaping to the atmosphere at the sides. Further more, a 2-3m insulated skirting and water impermeable layer around the house will be fashioned so as to funnel heat up towards the concrete slab. You want to prevent water soaking into the earth as that will take heat away with it as it falls deeper down into the water table. It is also unsuitable for areas where the water table is high for the same reason.
If the skirting is wide enough, it will take longer for lateral thermal conductive heat transfer compared the vertical and you will get back a generous percentage of energy that you dump into the ground when you need it most (in winter). At the start of summer when the heat is starting to escape the sides more, you don't need the geo-solar assist as much. Note also that the ground temperature is relatively constant year round, and can be in the vicinity of 15 deg celcius. It is less of a chore to raise the temperature of that thermal mass to something more comfortable - like 18-20 deg celcius to contribute to the house's heating requirements.

There are only a handful of examples of houses having done this, and usually they are located in countries where the summers are extremely hot and the winters extremely cold. This might not be worth doing in Motueka where the weather is relatively mild even in winter. Some thermal modelling of this system seem to also indicate only moderate gains of about 2-3 degrees C in winter (after equilibrium is reached) which is poorer than anticipated. However the actual implementations of the AGS system seem to indicate much better results. It is difficult to isolate the gains of AGS away from other complementary systems working in tandem in these houses.

It is important to note that both radiative and condutive heat transfer can occur independently of orientation (it doesn't necessarily have to rise). Only convection in fluids will rise to the top - so the AGS system, being based on earth (which isn't a fluid), is really a system of controlled radiation and conduction transfer, the latter being the dominant method.

Link to AGS:

Active solar:

The alternative to a passive solar house is one based on active elements. Although these systems tend to complement each other, active solar does add a layer of complexity to the mix. In a typical scenario this involves having a heat collector - which means either a flat plate or vacuum tube roof mounted panel that is of a sufficient size. The fluid is passed through and heated by the heat collector panels and this heat energy is stored in a large thermal battery - typically a larger than normal insulated water tank.

The most elegant solution I have come across so far for a thermal battery storage is the Latento hot water buffer tank system. This is very well insulated 500L water tank which has a variety of tubular metal coils which either carry thermal energy in, or carry thermal energy out. Energy is input from the roof panels and exits via the coils feeding a system of underfloor pipes (called hydronic heating) and also for the domestic hot water taps. The pipe loop that is used for domestic hot water is corrugated to create a highly turbulent water flow - this enables it to pick up heat from the water bath much more efficiently, compared to a straight pipe which has laminar flow in it.

The Latento tank cutout

Water coils in a Latento tank
The added secret sauce is the latent heat material (basically wax) which converts to a molten liquid from a solid at about 60 deg C. This acts as both an insulation layer to the top of the tank, as well as being additional thermal storage in the form of latent energy - or energy used to convert the material from a solid to a liquid. For the material to become a solid again it will have to release the heat energy within. As this occurs at ~60 deg C it acts as a thermal regulation layer.

It is specified that this tank can hold 50 kwh of thermal energy, and only looses 0.1 deg C per day - an impressive result. The main disadvantage is that is has a premium price tag which is around NZD2000 dearer than other more common systems.

A common alternative is to have a large buffer tank and a smaller domestic hot water cylinder. An 800-1000L insulated copper buffer tank will cost around NZD3500, and a 270L tank with a single coil around NZD$1500. A thermostatically controlled pump is used to transfer the hot water from the buffer tank to the domestic hot water cylinder which has a hot water coil in it. The buffer tank is open vented (not pressurized) and is able to take much higher temperatures giving it a superior thermal capacity for less cost. However the losses are likely higher as the insulation is thinner and heat is invariably loss in the transfer between tanks. The water in the buffer tank can be used directly in the undefloor heating as well as the solar heat collectors without additional piping coils which can save further costs. Solar heat collectors based on glycol will need a separate heating coil however to prevent it mixing with the water.

Most of these systems either have radiators (need higher temperatures) or hydronic underfloor heating. They usually have complex controllers to ensure proper heat distribution to each room - some systems give you the ability to preset the temperature for each controlled room or area. The hot water from the buffer tank is distributed to water circuits that are seperated for each room - some rooms will even have several loops in parallel. The hot water is distributed in a hot water manifold which has flow regulators and sometimes insulated hot water electric solenoids controlled by a smart controller. Usually a single diaphram circulation pump is used to pressurise the manifold system.

As you can see, an active system certainly complicates things substantially. This unfortunately can lead to added costs and potentially a penalty on operational longevity.

Our house thermal design:

What I see us doing is to stick with a passive solar slab design with the addition of a fan to help force the air around if the natural currents aren't sufficient to capture the heat readily. I still need to research how a rib/raft slab (for stiffness and strength) can marry nicely with the solar slab design.
However I'm also keen to put the hydronic underfloor pipes in (a relatively inexpensive exercise) even if we don't end up using it right away. The reason for this is obvious - you can't easily go back and do this part of the process later.

We will have solar hot water heating, but it will likely only be for a small domestic tank initially. Subsequently we will get a buffer tank and hook up the wet back from the wood cooker to it and pipe the solar hot water into it.

Later again we will hook up the underfloor piping to the buffer tank via a manifold, but use simpler and cheaper techniques for pumping water around the various circuits - this is yet to be fully considered. Perhaps instead of a manifold with flow regulators and solenoids, I will instead use small water pumps on each circuit - controlled by an Arduido type circuit?

Watch this space ;)

ps: I've just discovered Organic Rankine Cycle engines - but there is way more to learn about that before I blog on it.

Popular posts from this blog

Charry Charry Night

Adobe.. dobe ... dobe