El-Sitgeti-Electrical point 3-Yurt

El-Sitgeti-Yurt- Charging a Ni-Cd battery and heating up water with a DCV source 

The Yurt is the point of where most household appliances are used. Both ACV 230 and DC BUS are distributed here. The ACV 230 comes from point 4, which will be described in a later post.

The DC BUS is used here for 2 purposes:

1) Charging the Ni-Cd battery set

The charger consists of a simple SMPS bougth from ebay. They are sold as LED strip power supplies, and they are sold in various current ratings. We used here a 50 A (600W) type (50€), which is more than sufficient as the charge current will hardly go over 30 A. However having a current rating twice the nominal current will increase the longevity of the power supply and avoid over-heating. The output voltage is controlled by a potentiometer and it is adjusted to a value slightly above the float voltage as to provide for a maximum charge current of about 25 A and minimum water loss when the batteries are in full charge. Since the power supply does not have a feedback control loop the output voltage gradually increases according to the DC BUS voltage and thus with the solar irradiation intensity  (e.g. when the sun starts shinning DC BUS is low, and thus the charge voltage remains below the float voltage), and starts pumping current into the battery only when power starts being available from the panels.

The Power supply is wired through an electrical box that combines AC and DC parts. The voltmeter allows monitoring both the DC BUS voltage and the charge voltage by simple pressure on the left switch (see picture). The current value shown on the ammeter is the current consumption on the BUS, but the charge current can be easily calculated from that value.

Electrical schematics of the box. Note than when the switch is actuated to monitor the charging voltage the ground of the secondary part of the SMPS is connected to the ground of the DC BUS (primary). There was no way around this (using a simple switch at least), but we believe it does not raise any safety concerns as the time when the two grounds are connected only last for a few seconds.

View of the electrical box and battery charger. Inside view of the box and view of the charging voltage being monitored (pressing the left switch).

2) Heating up water with an IGBT controlled boiler

This is the first appliance of the blog to be modified to work directly on the DC current rather than on AC. If you read my previous posts you will easily understand why it can work.
Boilers are mainly composed (at least the most basic of them) of a mechanical thermostat (or thermal switch) and a heating element, practically a resistor with a resistance a few tenth of ohms (typically of a rated power of 1.5 to 2KW). The heating element is specifically designed so that its maximum current rating is achieved under a 230V RMS voltage. If we were to connect that heating element to the DC 350V it would simply overheat and eventually be damaged because too much current would pass through the resistor. The way of avoiding this is to chop our DC source at given frequency and to modulate the pulse width (or duty cycle). Since the RMS value of any variable voltage is given by the integral of the wave over its time period (see figure below), by feeding our heating element with such pulse width modulated (PWM) signal, with an amplitude of around 350V (the value of the DC BUS), we will actually be varying the RMS voltage value we are applying to the heating element. By making sure the RMS value doesn't go over 230V we will ensure the heating element will not withdraw more current than its actual rating.

RMS voltage of a PWM signal respect to its duty cycle. As seen in the formula, the RMS value is proportional to the square root of D which corresponds to the duty cycle.

We can even extend this concept further, by actually varying the duty cycle not only to limit the RMS value, but to limit the current consumption to that available at that particular moment in time. That is, if solar irradiation is low (and therefore the available current on the BUS is low) we would decrease the duty cycle as to adjust the current drawn from the heating element to the available current from the solar panels at maximum power point (MPP). This will have the effect of decreasing the temperature of the heating element though, but will still offer the advantage of heating water however at a slower pace. If the duty cycle was to be fixed to an RMS value of 230V, the boiler could only be turned on a sunny day at around noon, for it not to drive the voltage of the BUS lower than the MPP voltage.

To achieve such task we designed an boiler controller device with the above-mentioned characteristics (120 €). It consists of an op-amp comparator which compares the voltage of the DC BUS to a threshold set by the user (by means of a potentiometer) and an IGBT driver which feeds the gate of a power IGBT with a square wave PWM signal. the IGBT driver varies the duty cycle so that the value of the DC BUS does not drop bellow the set threshold. Note that the voltage threshold is an indirect way of limiting the current passing through the heating element. It is not as accurate as actually limiting the current to a set value, but it does the job as the voltage threshold will be set to a value close or above the MPP voltage, and therefore no more current than that available at MPP will be drawn from our panels.

Front view of the boiler controller.

The device comprises a digital thermal switch which turns on or off the heating depending on the temperature threshold set by the user. Thus, the boiler was stripped down completely and the original mechanical switch was completely removed and replaced by the temperature sensor that came with the digital temperature switch (a 5Kohms NTC resistor). It is as simple as that! And in practice it works extremely well: With a 50 liters boiler (only 65 € in our local hardware store!!!) and with  the temperature set to 67.5°C, it only takes between 30 to 45 min to heat that volume from 40°C to 67.5°C early in the morning (the heating element has a 2 KW rated power). And the beauty of it is that it reaches that temperature even on the cloudiest weather conditions! It just heats up the water more slowly. The process is fully automatized so you never have to worry whether you are going to have hot water at the end of the day. As long as the DC BUS rises over the set threshold it'll start heating up and adjust the heating rate according to the current available on the BUS. The threshold is set according the the priority we want to give it. If it's set to a low value it will have a high priority and lower priority if it is set to a high value.

View of the yurt connection point (3), with the electrical cupboard at the bottom, and the boiler (50 L, 2 KW) on top of it. The right picture shows the boiler controller in the cupboard. The DC BUS input is wired through a 10A fuse to protect the heating element in case the IGBT gets short circuited (nominal current at 230V RMS voltage is approx. 8.5 A).

No more cold showers!!!