Monday, September 27, 2010

Weeks 17 & 18


This may be the last posting before the final one, upon completion, now estimated to be in two and a half weeks.  Not much has been happening outside except for the start of some entry stair construction and the sealing of the natural pine ceilings of the solarium and the entry way.  Inside, however there has been much activity related to door installation and door and window trimming.  A first coat of paint has been applied by spray gun and the cabinets have started to be hung.  The cabinet appearance is funky because there was no attempt made to match adjacent pieces as to shade and structure.  It looks like a Cubist work of art - the glass is half full.

The electricity has been turned on and the electrical heat is working, currently being used to help dry out the concrete floor.  The picture shows electrician Steve, plumber, Randy and electrician Nick.  In the background you can see.......the 10 kW heater, expansion tank, and pump.  Not visible are the manifolds for the three zones (bedroom, main, and half of basement), the zone valves, the low water detector to protect the heater and many ball valves and faucets for maintenance.



The Yurt Hunt
Last week I participated in a Yurt hunt, that is to say, we went to Red Hook, NY and disassembled a 5 year old, 24 ft. diameter Pacific Yurt and hauled it back to our community to serve as an interim "Common House".  Although it was tricky disassembling the yurt, most of the work was disassembling the deck supporting it.  Yurts are great for milder climates and not too easily adaptable to colder, windy climates so we may have a few weeks where it just won't be worth trying to heat it.  With walls that are leaky and only have maybe an R5 insulation, the pellet stove will have to work hard on those windy below zero evenings.

The Solar Energy Collector Choices
In the last post it was estimated that without wood heat that I would use at minimum 6700 kWh of energy per year after subtracting the passive solar contribution.  So how can we capture that much energy with active solar?  

Many say that solar thermal is the best way to capture solar energy in the form of heated water.  But what is the complexity of a solar hot water system and how well does it perform?  Many also say that solar photovoltaic is inefficient and too expensive.  Let's look at the numbers for residential systems.

For both types of collectors, it should be noted that they are rated for the sun at full intensity shining at normal incidence, i.e. perpendicular to the collector surface.  Any deviation in angle from normal will lead to a greater proportion of the sun's energy being reflected from the transparent/translucent glass surface which protects the collector.  Thus fixed collectors cannot be as efficient as single or double axis sun tracking collectors.  Up to around 40% of potential sun energy is lost due to reflection experienced by fixed collectors.

A solar thermal system usually consists of one or more black body collectors which heat water.  When the water reaches a temperature greater than the destination temperature, a pump transfers the heated water, in most cases through a heat exchanger at the bottom of a large water tank.  A heat exchanger at the top of the tank can be used to supply heat from a backup source, be it electric or gas.  The heated water in the tank can be used for either washing or heating a floor.  A simple system consisting of one collector, tank, pump, differential temperature control and plumbing can cost in excess of $6K, I was told by one supplier.  Long periods of cold and overcast skies will require a backup heat source, yet possibly an extra expense.  Panel efficiencies are usually around  70% at normal incidence for either flat panel or solar tube collectors.   Considering that they are not pointing toward the sun at all times during the day, they are only receiving around 60% of the potential energy they could receive thus giving them in reality around 45% efficiency.

A solar photovoltaic system usually consists of one or more PV panels mounted either fixed on a roof   at an optimal angle or on a sun tracker.  The direct current (DC) produced can then be stored in costly batteries for off-grid usage or be changed to alternating current (AC) by an inverter for connecting to the grid and thus using the grid as a bank account.  This assumes that one is allowed to tie in by the local utility and assumes that the utility pays a reasonable rate for electricity received.  Installation of fixed panels on properly oriented roofs requires penetration of the roof's surface for the panel mounts and the wiring.  Other considerations for roof mounted systems are leaf debris and snow accumulation on either side of the panels and the beneficial heating of the roof in the winter months.  Trackers are complete systems that can be easily inserted into rigid soils with a small front loader.  Trackers are more complex mechanical and electrical systems with some using GPS for tracking and cell phone connections for monitoring and on-line reporting of performance.  A 4 kW tracker will have 20 solar panels, each capable now of operating at its maximum efficiency which may be around 17%.

Purchasing alternative energy equipment is currently supported by rebates and tax credits, not unlike the estimated over $500 billion support of fossil fuels worldwide (Where is this free market?).  For example, a 4000 W system, capable of generating around 5640 kWh annually in Vermont would cost $31,000.  After a $4000 rebate from the Vermont Renewable Energy Resource Center and a 30% federal tax credit, the final cost is $17,700 installed.

In the co-housing community in which I am building there is the opportunity to place the collector in the lower more wind-protected, yet wide open meadows rather than in the tight residential area.  This my plan and it will be appreciated by the sheep grazing there as they are always seeking shade in summer.  Now where is that pot of gold the rainbow last week was pointing to?


















And finally a whazzit puzzle found around the construction site:

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