House Computers

(20150620:175911) The main house computer is a BeagleBone Black, and runs the heating program and logs relevant environmental values. This machine is known as bastille on the local network, and is not available outside the house network.

This system has a bit of history behind it. It was originally designed to run on an Intel 386 box (back in about 1999) known as central, but that machine has been decommissioned (its main disk died), and all its functionality was moved to two distinct machines known as garedelyon (a small unix box) and ringwood (an old MacBook Pro), together with an Arduino driving all the relays which controlled the house functions. flinders (a larger server machine) was used as a site server for the domains ajh.co and ajh.id.au, which were used as the web interfaces to the system.

Unfortunately, ringwood died in Jan 2015 while we were overseas, and took out several other systems as it went. When we returned in Mar 2015, I had a couple of BeagleBone Blacks, so it seemed natural to use one of these as a replacement. Given the hobbyist nature of the BBB, it could easily replace both systems, and revert to the single machine functionality of the old central system. This machine (known as bastille) now performs all the data logging and control functionality, except for the Arduino relay functions, and even that is to be migrated back to this new central house server.

Descriptions of the functions performed by this machine is being developed in a separate page, and a literate program describing the suite of programs used is at the HouseMade page.

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Solar Photo-Voltaic Array

We now (from 1 Oct 2013) have a smart meter. I will not at this stage enter into the politics of this, other than to say that it was a great opportunity. I am trying to make the best of the limited data available, but it is not as easy as it might be. Hopefully there will be further displays on this page to show what I have in mind. In the meantime, as mentioned above, the displays shown below will not be updated until the house computer is re-instated.

The first two plots here show watt hours generated per day by our solar panels and grid interactive inverter. The plot to the left shows the daily cycle for today and the last 7 days, while the second plot below shows the total daily energy every day since installation. Clicking each plot will show more detail.

The daily plot is restricted to the hours of 6am to 6pm, since input is negligible outside these hours. You will note the stepped nature of the plot - every 30 minutes the output appears to change slope, and output appears to limited by some factor other than insolation. I have not been able to find any explanation for this - there is nothing in the manufacturer's manuals, nor their web pages. If anyone can shed any light on why this occurs, I would be pleased to hear from them. My best guess currently (sorry!) is that it is some battery recharge regime.

There is a slight liberty taken with the data in the second plot. This was due to equipment failure over the summer of 2004/5. Data from the previous summer is substituted to maintain the shape of the graph. The daily plot is generated by our house computer, and updated automatically every 10 minutes. The long-term plot is generated manually (last update 20111217).

The solar assembly consists of 20x75W panels (1500W total) feeding into a Trace SW3024 E 3300W inverter system, buffered by 4 Sonnenschein 6v 330AH gel cells (7920WH). In sunny weather, the system generates enough power that the meter is usually spinning backwards! Two of the house power circuits (including the refrigerator and all computers) are fed from this inverter, giving about 24 hours of autonomy (at normal current levels) in the event of grid supply failure. (see footnote *)

Our current electricity drawdown from the grid is about 14KWh per day (averaged over the year), and the solar system supplies an average of 4.6KWh (see chart above). That gives an average fraction of about one quarter of our electricity needs being supplied by the sun. That translates a saving of approximately $0.84 a day (at 18.3c/KWh). The system cost about $6000 ($4000 for the inverter, $2000 for the rebated solar panels, not counting the batteries, which are only for UPS protection), and will therefore pay for itself (assuming no increase in electricity costs, an unrealistic assumption) in about 25 years! But of course, we are also saving about 2.5 tonnes of greenhouse gas a year, (roughly) the equivalent of not driving our car anywhere for the year.

With recent rises in electricity costs (as predicted), here is the actual payback chart as at the beginning of 2014:
Date Cost $/KWH Amortised Cost Break Even Date Years to BE
10 Sep 2003 0.13 6000 28 Feb 2031 27.5
12 Jun 2009 0.15 4743 6 Apr 2028 18.8
10 Feb 2010 0.183 4575 28 Dec 2024 14.9
12 Nov 2011 0.1995 4037 16 May 2025 13.5
18 Jan 2012 0.2156 3982 18 May 2024 11.0
1 Jan 2014 0.29 3304 11 Aug 2021 7.7

Rainwater Harvesting

We also have a rainwater system consisting of two 2250 litre Plastanks, fed from the house roof, and pressurised by a 240v Onga Riva-Flo TF30 centrifugal pump. This pump delivers 30 litres/minute at 8 metres head, and is quite adequate for our purposes. Delivery is to two toilet cisterns, and to a garden watering system (8 solenoid operated outlets and 2 standpipes). The garden watering system is driven automatically by cron jobs running on the house computer.

The plastanks are supplemented by a 4000 litre 'wootank'. 'Wootanks' are tanks made of wood, and this one is home-constructed from 12mm plywood and 100x50 hardwood. The hardwood is used to build an 'exoskelton', and the plywood is used to make a box shape, lined with a pond pool liner. This tank captures water runoff from the rear of the house, while the plastanks capture water from the front of the house, and there is a 3m difference in height of the base of the tanks. A submersible pump, non-return valve and separate ball valve allow water to be transferred in either direction between the tanks. In addition, overflow from the upper (plas-)tanks is piped into the wootank. Overflow from the wootank tops up the garden ponds, and overflow from these goes to waste.

On the left is a plot of the water level in the rainwater tanks monitored electronically by means of a water-capacitance oscillator bridge. At the moment, only 1 tank is monitored, the system has been calibrated in terms of total plastank capacity. Full supply level of approx 4000-4500 litres is achieved by plumbing the overflow back into the tank through a 'snorkel' type fitting, which is adjustable. This allows the overflow level to be significantly about the fitted overflow outlet. Currently, it is set to minimum, since this extra head uses the domed section of the tank, which suffers from the problem that overflow in heavy rainfull conditions is non-linear, and the overflow in fact does not cope! You can see this in heavy rain conditions, as the level rises above the overflow, but drops back once the rain eases off. In really serious conditions, the tanks themselves overflow, which is not good!

Below is a plot of the same data as the left diagram, but drawn over 8 days, rather than 1.

Temperature Maintenance

On the right is a plot of the inside and outside temperatures of the house. You can see the effect that the heating system has. The data is collected from a Dick Smith wx200 weather station and logged through an RS232 interface. The heating is controlled by a relay driven from the house computer (which also generates these plots). The cycling of the "thermostat" (There is no such animal, it is all soft controlled) can be clearly discerned when the heating is in operation. The actual "thermostat" temperature is controlled by a cron job, which can be programmed (or overridden) through a web interface.

For summer, we have an evaporative air cooler installed. At the moment, this is turned on and off manually, according to comfort. The purple line plots the outside dew point. This is the theoretical minimum achievable by the evaporative air cooling, and gives a guide to how effective the cooling will be when on. A high dew point equates to lots of humidity, and hence a loss of effectiveness of the cooler. Note that the dew point does not necessarily follow the outside temperature.


(footnote *): The irony of this is that while my wife and I were away on a week's holiday, my son and his band cronies plugged guitars and amplifiers into the UPS circuit, and played so loudly that they overloaded the inverter, tripping its overcurrent protection, and turning off the fridge. He simply moved to another power point and thought no more about it, until he noticed all the icecream running out the bottom of the fridge! We came home to a disaster! Just goes to show, such systems are not 'fool-proof'!

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