emonTx single AC power supply - Part 1 Simulation

The current emonTx V2.x can monitor AC voltage, real power, power factor and direction of current flow, this is possible by using a plug-in 9V AC-AC transformer to provide a stepped-down AC voltage waveform. See Measuring AC Voltage with an AC to AC power adapter in the building blocks section for more details. The emonTx also requires an additional (separate) DC power source, this is usually provided by a 5V USB plug-in adaptor.

Obviously it's desirable to do away with one of the power supplies and to use the AC-AC adapter to provide both the AC signal and the DC power to the emonTx. Early tests indicated that using the AC signal to provide a DC power supply adversely effected the quality of the AC signal and therefore the voltage and real power reading of the emonTx. Full wave rectification with a bridge rectifier creates a power supply with floating supply rails and loading the AC-AC adaptor caused distortion of the AC sine wave sample signal.

There has been an interesting form thread on the topic of using a single AC power supply for the emonTx. There were number of suggestions for circuits to achieve this. Many of the circuits suggested looked feasible but some added considerable complexity and components such as centre-tapped or isolating transformers. We are keen to use an off-the-shelf certified power supply unit wherever direct connection to mains power is involved. 

Recently Robert Wall contacted us with a single AC power supply circuit design based on a half-wave rectifier and LDO linear voltage regulator. The circuit only requires an additional two capacitors, two diodes and one resistor to supplement the components we already have on the emonTx PCB. Robert has been very helpful offering his expertise and experience as an Electronic Engineer to the project. The technical testing report on the SCT-013-000 CT and Mascot AC-AC adapter in the building blocks section are all Robert's work. 

Note: this circuit design is still in development, we are currently at the simulation stage

Fig1. Half wave rectifier AC-DC power supply and AC voltage sample 
The key to Robert's circuit is current limiting resistor R1, this limits the current draw from capacitor C1. The resistor value was chosen to attempt to charge C1 gradually over the positive half cycle. See the labelled waveforms below to see how this works in practice simulation! C2 is a de-coupling capacitor and R3-R5 reduce the AC sample to the correct magnitude and bias ready to be sampled by the ATmega328 ADC. 
Fig2. Simulation waveforms under normal operating conditions
To demonstrate the effect of the current limiting resistor R1 the simulation was ran with R1= 1 ohm, see the labelled waveforms in Fig.3 below. The capacitor C1 now charges up quicker drawing more current from the AC-AC adapter, this loads the adapter creating a 'dip' in the AC sample waveform. This will negatively effect the voltage and real power reading on the emonTx. 
Fig. 3 Demonstrating the effect of low value current limiting resistor R1
Simulation has indicated that with a current draw of about 8mA (ATmega328) and using the Mascot AC-AC power adapter with a mains voltage of about 240Vrms results in a -6mV difference in the Vrms of the sample waveform when compared with a perfect sine wave. This should be small enough to not adversely effect the AC voltage and real power reading, a tweaking of the calibration will probably be required.

The next step is to move beyond the virtual; build up a prototype to see if it performs in practice like the simulation suggests!

I've posted the LTspice simulation files used to generate the simulation waveforms on the forum: http://openenergymonitor.org/emon/node/704

View your power data as a histogram in emoncms

This blog post details how to view your power data as a histogram in emoncms. This is a really useful feature implemented in emoncms by Paul Allen http://wattdata.blogspot.com  (MarsFlyer on Gitub: https://github.com/MarsFlyer). So a really big thank you to Paul for this!

The histogram shows the energy (KWh) used at a particular power (watts) which allows analysis of what types of devices (low / medium / high) are using the most energy. 

Here is the histogram visualisation for my house, embedded directly from emoncms.
Click on a particular day to see a histogram of that day:

Viewing your power data as a histogram

1) Send power data to emoncms see here for details of doing this with an emontx and emonbase: see Build a home electricity energy monitor

2) Click on the power input to bring up the input configuration page. Set up processes as in the screenshot, one power process, one kwhd process and one histogram process:

3) Click on Feeds to bring up the feed/list page, click on the histogram feed:

4) In the histograms feed/view page click on the graph type histogram:

5) Once you have been logging histogram data for a few hours you will start to see histogram bars form at the different levels of power that you use. This histogram graph shows energy used at a particular power for all time. So is a really useful tool for measuring what is the total energy used by the electric immersion water heater for example, we've used 62 kWh @ 3500W since April 13th.

6) To view your histogram data at a daily basis as in the first embedded visualisation above enter your feed id's and apikey in the following emoncms visualisation url:


All the source code for the histogram functionality is available in the main emoncms3 repository on github http://github.com/openenergymonitor/emoncms3

Paul Allen has achieved some remarkable reductions in energy use over the last 4 years and uses a version of the histogram beautifully to visualise the changes, see his blog post here: Personal experiences of saving 42% on my electricity use

Measurement of heat loss in a hot water cylinder

Carrying on from the last blog post about measuring heat flux in and out of a hot water cylinder.

Heat loss from a hot water cylinder should be proportional to the temperature differential between the inside and the outside of the cylinder by a constant: Watts of heat loss per kelvin. See heat conductivity.

Watts of heat loss can be calculated from the temperature reduction per unit time of the average cylinder temperature. If we plot heat loss against average cylinder temperature we should get a linear relationship where the slope of the line of best fit is the heat loss factor, see graph 2 below.

This constant the heat loss factor for a particular cylinder is a useful measure in evaluating the performance of a cylinder.

My hot water is heated with a 3kW electric immersion heater. I wanted to know how much heat would be lost if I leave the immersion heater on all the time at a given temperature and so how much energy I could save by only putting the immersion heater on specifically when its needed i.e for a bath in the evening.

The results

I have selected a period of time here where both hot water and the immersion heater where not used. Leaving only solar hot water heat input and heat loss from the cylinder as the only drivers of the system. The solar hot water coil is in the bottom of the cylinder which heat's the cylinder relatively evenly compared with the immersion heater making it ideal for this test. I have filtered out all positive heat gain so that we can see the heat loss clearly:


The conclusion is that we are saving about 1.6 kWh/d from reduced heat loss (30C temperature difference over 23 hours), which is a good saving from something relatively easy to do. It may be that we are saving more than this from reduced hot water demand in the first place by not having hot water on tap all day but this is another thing to test.

If your monitoring hot water cylinder temperature It would be great to see how these figures compare.

Download the source code: heatloss.zip

Measuring heat flux in and out of a hot water cylinder

The heat input into a solar hot water cylinder is usually measured using a flow meter and two temperature sensors. Flow meters are quite expensive and require plumbing work to install. At OpenEnergyMonitor we're all for non-invasive monitoring methods! So..

Can the heat flux in and out of a hot water cylinder (a fixed volume of water) be approximately calculated by measuring the average temperature change in the cylinder in kelvin per second and then multiplying by the volume of the cylinder and the specific heat of water?

This method may only require two temperature sensors so will be considerably easier and more affordable than installing a flow meter.

I've used an emonTx with two DS18B20 temperature sensors one positioned in the temperature sensor sleeve that is in the bottom half of the cylinder and the other positioned in the temperature sensor sleeve in the top half of the cylinder. The emontx is also monitoring the solar hot water collector temperature, controlling the solar hot water system and monitoring house electric consumption and solar pv electricity generation :)

Calculating heat flux
Heat Flux (Watts J/s) = Specific heat of Water (4186 J/kg/K) x Volume of cylinder (Litres) 
x temperature change per second (K/s)
The result
The heat flux is the blue plot and average cylinder temperature is the red plot. I have selected a period of time where the electric immersion heater is on. Feel free to explore the data by using the zoom and pan buttons.

I'm surprised by how good the initial results seem to be. I know that the electric immersion heater consumes about 3400W from my electricity monitoring and the heat flow input calculated via the above method gives pretty much the same power input plus or minus a few hundred watts or so.

Questions for further development
What is the minimum number of temperature sensors needed and what are their optimum positions to give best results? What is the effect of stratification?

Download the source code: hotwatercylinder.zip

Emoncms multilingual support

The latest update to emoncms adds a basic implementation of multilingual support.

Adding a language

1) Make a copy of the folder Views/en and call it the language your creating a translation for, i.e cy for welsh:

.. or use a language that has already been translated from the emoncms_languages repo, i.e cy...
https://github.com/openenergymonitor/emoncms_languages (feel free to send a pull request with your translations)

2) If your creating a new translation, translate each view script inside your language folder.

3) Add your language to the language options in index.php:

Change the line:
$lang =  $_GET['lang']; if ($lang=='en') $_SESSION['lang'] = $lang; else $lang = null;

to (with your chosen language code)
$lang =  $_GET['lang']; if ($lang=='en' || $lang=='cy') $_SESSION['lang'] = $lang; else $lang = null;

2) Add your language to the user_view language selector.
Until a better implementation is created, this needs to be done for each language.
In Views/en/user_view.php

add (or your chosen language)

<option value="cy">Welsh</option>

<option selected value="en">English</option>

Repeat this for the user_view that belongs to the language your adding and set the selected attribute to the added language.

Any thoughts on a better way of implementing this is as always welcome.

emonGLCD Solar PV PWM LED Proportional Indication

The tri-colour LED's on the emonGLCD V1.3 are connected the the hardware digital PWM lines on the Atmega328. This allows the brightness to be smoothly controlled in 0-255 steps.

Up-until now we have not made use of this feature.

The video below shows a demo of Solar PV generation and power consumption ramping up and down. The tri-colour LEDs smoothly increase in intensity green or red depending on the amount of power being imported or exported.

This should serve as a nice at-a-glance visual indicator to how much power is being imported or exported at any given time. At the moment the example has been configured so that the LED's are at their brightest when 4kW's are being exported or imported. In the future it would be great if the display learnt what size PV system (kWp) is installed.

It would be possible to implement a similar feature on the Home Energy Monitor display to indicate the level of power being consumed, but this will have to be done carefully not to annoy the user! The problem with this is what constitutes a high level of consumption? To be useful the display needs to 'learn' what's high and what normal level of consumption.

Thanks to Rob Walker and the discussion on this thread for help in implementing this. Rob has also implemented proportional control of the white LED backlight level based on the light level in the room measured by the on-board light sensor (LDR)..nice!

All these changes and some other minor improvements have been committed to the emonGLCD github repository: https://github.com/openenergymonitor/EmonGLCD. Please test and report feedback on the forums.

Update: The single CT Home Energy Monitor emonGLCD example sketch has been updated to also include proportional red LED PWM brightness control corresponding to the amount of energy being consumed.

Embodied Energy of Electronic Enclosure Materials

For us the underlying aim of energy monitoring is energy reduction. It's therefore important that over its useful life an energy monitoring system contributes to the saving of more energy than it involved in the embodied energy of it's production.

Recently we have been having the discussion as to which material is environmentally better to use for electronic enclosures.

So I did some research:

Caution: The figures below were obtained from reputable sources but should not be taken on face value.
  •  Extruded Aluminium takes approximately 40-80% more energy (KWh/Kg) to produce than ABS plastic [1][2] *
  • Aluminium is widely and easily recycled (even if its been anodised [2]) and the recycled product is of just as high a quality as the original. ABS plastic can be recycled in some locations but the recycled product is of much lower quality than the original, 'downcycling' would be a better term! 
  • Recycled aluminium takes about 30% less energy (KWh/Kg)  to produce than ABS plastic [3].
  • Plastic is made from oil which is a non-renewable fossil fuel.
  • Aluminium is the most abundant metal in the Earth's crust, extracting it requires lots of electricity which could be generated from renewable sources. 
With current processing and recycling methods we think aluminium could be the best material to use, it does take more energy to produce in the first place but once produced it can be recycled again and again to make new things with little extra energy.

What do you think?

[1] http://www.inference.phy.cam.ac.uk/withouthotair/c15/page_88.shtml
[2] www.victoria.ac.nz/cbpr/documents/pdfs/ee-coefficients.pdf
[3] http://www.worldchanging.com/archives/005656.html

*Aluminium is about 2.6 times denser than plastic therefore a similar case made from plastic could be lighter. Further research is required before drawing a conclusion. 

Engineers thermometer

I have just configured an emonGLCD and a battery-powered emonTx unit with 3 temperature sensors. This would make a useful heat-pump engineers tool, and probably has many other uses.

The emonTx has 2 probes (DS18B20's) on 2m cables, and one on-board the PCB. It's battery powered so can be put anywhere in range. The emonGLCD display shows the 3 emonTx temperatures and also a local temperature.  Temperatures are transmitted via wireless from the emonTx to the emonGLCD every 2 seconds.

Typical uses would be for balancing radiators. i.e T1 on the flow, T2 on the return. I have added T1 - T2 (delta T) because that is what you are often looking for.   It could also be used for air-in, air-off off an air source heat pump, or and flow/return situation.   It would make a good tell-tale device. i.e. to alert if any system goes out of its best operating ranges.

I also have the temperatures data going to a Nanode emonBase to be logged to emoncms so that graphs can display temperatures over time. You really need this to balance up central heating radiators. i.e. fix it to a different radiator every day and compare results. The flows can be balanced accordingly.

The DS18B20's are not ideal because they are a bit chunky (as opposed to thermocouples). On the other hand, the accuracy is excellent and it seems that the values can be relied upon. I have chosen to pot-up the probes myself.

They are fragile, and the ready-made sensors from the OpenEnergyMonitor shop are much more rugged and can easily be tie-wrapped to a copper pipe and insulated etc.

Next thing is to battery-power the emonGLCD so it becomes a portable tool.  

Another addition would be a LCD pulse input. This would give me a reading of kWh, and would save me using a stop-watch, as I have many over the years.

Adapting these devices could get addictive!!

By John Cantor heatpumps.co.uk

A short video of OpenEnergyMonitor Labs

Timelapse footage from inside and outside OpenEnergyMonitor Labs here in the mountains of Snowdonia, North Wales, UK.

We had lots of fun making this. If only each day was longer!