Solar PV Monitoring System
This page documents how to build a wireless web-connected solar PV monitoring system that monitors both generation and grid import/export using the emonTx, emonGLCD and emonBase modules.
The solar PV monitoring system consists of three hardware units which communicate wirelessly: emonTx – transmitter unit, emonBase web-connected base station unit and emonGLCD -living room display unit. These three units are the standard building blocks of the OpenEnergyMonitor end-to-end open-source monitoring system. All these units are open-hardware and Arduino software compatible.
emoncms v3 open-source web-application is used on a remote web-server for web-based logging, graphs and a real-time dashboard.
Two clip-on CT current sensors and an AC-AC plug-in voltage adapter are used to sense the solar PV generation and consumption. It is possible to sense the LED pulses from a pulse output utility meter to monitor the power flow, however CT sensors give a much better quality reading of instantaneous power. When monitoring power using pulse counting method the sample rate is limited to the number of pulses, at low power values the rate of pulses can be extremely long.
1. Install sensors & setup emonTx
How the CT sensors are connected installed depends on how the solar PV system has been installed. For sake of reference we have called these systems type 1 and type 2.
Type 1
When the generation and consumption can be monitored separately. The amount exported/imported to or from the grid is simply the excess or deficit of the total of generation minus consumption. Knowledge of the direction of the current is not required therefore a plug-in AC-AC voltage sensor adapter is not essential but still recommended for accurate readings. For a type 1 solar PV monitoring system the grid import/export is calculated as follows:
Grid (import/export) = Consumption – Generation
Consumption and Generation should be positive, reverse orientation of clip-on CT sensor if not. Grid calculation will be positive when importing and negative when exporting.
Type 2
When the generation and consumption cannot be monitored separately, i.e the AC output from the solar PV inverter is fed into a spare MCB in the fuse box. Other household loads such as lights, shower, electric cooker etc. are also connected to other outlets in the same fuse box. If this is the case the output from the PV inverter and the grid import/export connection will need to be monitored instead. Knowledge of the direction of the current is required to determine the difference between power import and power export. Therefore an AC-AC voltage sensor adapter is essential.
When the AC-AC voltage sensor is used (see below) the grid import/export CT reading will go negative when exporting or vice-versa depending on the orientation of the CT clamp round the wire. To be compatible with the software examples included with this documentation it is desirable to clip the CT round the grid import/export cable orientated so that the reading is positive when importing and negative when exporting. The correct orientation can be determined by trail and error.
Household power consumption can be calculated in software:
Power consumption = Solar PV generation + Grid import/export (negative when export)
AC-AC voltage sensor adapter
In order to determine the direction of the current flow (important for being able to tell the difference between grid import or export) an AC voltage reading is needed to provide a point of reference; this result is a positive or negative current current reading depending on the direction of the current and the orientation of the CT clamp.
The emonTx obtains a voltage reading using an AC-AC 9V plug-in transformer. Using this adapter also allows us to monitor the RMS AC voltage, real power and power factor. For information about how the AC-AC adapter and AC RMS reading works see relevant section in Building Blocks. A nearby power socket is required for the AC-AC adapter.
Note: The emonTx cannot currently be powered from this AC-AC adapter. Rectifying and loading the AC-AC adapter effects the sampling of the AC waveform. A separate 5V USB power supply or batteries must be used to power the emonTx. There is an ongoing thread on the forums discussing how this problem might be overcome.
CT sensor connections
General note regarding the installation of CT sensors: the clip-on CT current sensors must be clipped round either the live (brown in the UK), or neutral (blue in the UK) wire. Not both. It is sometimes necessary to carefully remove plastic ducting to access the live an neutral wires. Live terminals should not be exposed but if in doubt switch off the power before investigating. A shock from AC mains electricity can be fatal, if in doubt consult the advice of an experienced electrician.
The CT sensors should be connected to the emonTx before being clipped round a live cable.
Above: CT installation on the live wire AC output from the solar PV inverter. Generation meter can be seen on the top right
Powering the emonTx
The emonTx can be powered from 2 x AA batteries or via mini-usb. A 5V USB adapter (commonly available as mobile phone chargers, see this blog post before choosing an adapter). The power adapters we sell through the shop have been tried and tested with the emonTx and emonBase.
emonTx firmware
See here for a generall guide to uploading code to the emonTx: http://openenergymonitor.org/emon/modules/emontx/firmware
emonTx Arduino sketch examples can be downloaded from github: https://github.com/openenergymonitor/emonTxFirmware
The emonTx CT_123_voltage example is the correct example to use when using an AC-AC adapter. If your using a emonGLCD use channel 1 on the emontx for consumption and channel 2 for generation.
Arduino IDE 1.0 and a USB to UART serial programming cable is required to upload sketches to the emonTx.
2. Setup emonBase
Main NanodeRF documentation page
The emonBase receives the monitoring data via wireless from the emonTx and posts online to a remote sever on which an installation of EmoncmsV3 is installed, the OpenEnergyMonitor demo server vis.openenergymonitor.org can be used.
Ensure the emonTx and NanodeRF have the same frequency RFM12B transceiver module and that the correct length antenna is used. The correct frequency for the module also needs to be set in the software prior to upload.
The emonBase needs to be powered by 5V USB. See notes above in the 'powering the emonTx section' regarding choice of 5V usb power adapter.
The IP address for the NanodeRF is automatically obtained from the router via DHCP.
Download and upload the NanodeRF_Power_RTCrelay_GLCDtemp sketch.
Main emonGLCD documentation page
The emonGLCD was wall mounted in the living room. The purpose of the emonGLCD is to give the home owner an instant reading of house energy usage compared to the solar PV generation.
It has been suggested that a home owner might attempt to reduce their consumption at times when a low amount of power is being generated and perform non-essential tasks that consume more power such as washing machine, dishwasher etc. when excess power is being generated. To give the home owner a clear indication of excess power being generated the tri-colour LED's on the top of the emonGLCD are green when power is being exported (excess power being generated) and red when power is being imported (power deficit).
Financial benefits aside, it was found that the home owner felt personal satisfaction by keeping the lights green when at all possible. Behavioral change due to domestic solar PV monitoring is an interesting topic, there have been several research papers written on the topic, the papers highlight the importance of monitoring and clear indication of both generation and consumption to prompt changes in behavior. The monitoring system described here gives far superior user indication to those used in the research below
The emonGLCD receives monitoring data from the emonTx. The emonTx, emonBase and emonGLCD are all equipped with transceiver wireless chips. For best wireless performance ensure the emonTx, emonGLCD and emonBase have the same frequency RFM12B transceiver module and that the correct length antenna is used. The correct frequency for the module also needs to be set in the software sketch prior to upload.
The values for kWh/d displayed on the emonGLCD are calculated on the unit, in order to reset the counters at midnight each day the emonGLCD must know the current time. The current time is obtained from the internet via the Nanode RF. See this blog post for explanation on how this works.
Download and upload the emonGLCD_SolarPV sketch.
Power Consumption
The goal of this monitoring system and that of the OpenEnergyMonitor project is to reduce energy consumption through increased understanding of how power is consumed. The monitoring system itself has been designed to consume the lowest amount of power possible.
The combined power consumption of the three hardware units in the system is 6.5W (57kWh/year), this is less than most ADSL internet routers. For the monitoring system to be worthwhile it should save at least this amount of energy. The emoncms installation on a remote web-server is implemented as a cloud instance. Cloud computing enables severs to be better utilised, therefore less power is consumed per application than with a traditional VPS.
In order to save power the tri-color LED's on the emonGLCD turn off when the solar PV stops generating (when it's dark!) and the backlight turns off at night.
Web Software
web-based logging, graphs and dashboards
Main emoncms v3 documentation page
emoncms web based dashboard screen shoots:
See the lower section of the Home Energy Monitor documentation for information on creating a simple dashboard on emoncms: http://openenergymonitor.org/emon/applications/homeenergy
Acknowledgements
Like most open-source projects we rely heavily on the work of others. Thank you for sharing:
JCW - JeeLabs.org: RFM12, EtherCard, RTC software libs, JeeNode and GraphicsBoard Hardware designs and much electronics insights.
Ken Boak - Nanode and NanodeRF design and general advice and guidance
Andrew Lindsey: Ethernet library development - notably adding DHCP support
Miles Burton: DS18B20 temperature sensor library
