SunForte System I

Mounting Structure
Installed: SunForte Upgrade Kit (15 deg)
http://www.conergy.gr/en/Desktopdefault.aspx/tabid-1066/1286_read-8789/

Today our project reached its first milestone, with the completion of its first mounting structure - the SunForte! After enduring inclement weather in the previous week of installation, our team was back at Singapore Polytechnic to set up the four PV modules on the SunForte frame.

With the use of quickstones and module clamps, it was relatively straightforward to install the modules:


PV modules
Installed: Yingli Poly-Si 110 Wp
http://www.yinglisolar.com/images/110%EF%BC%881470%EF%BC%89%20.pdf

Product Specifications
Nominal module voltage: 12V
Max Power: 110W
MPP voltage: 17.5V
MPP current: 6.3A
VOC: 22.0V
ISC: 7.0A
Module efficiency: 11%
Two PV modules are connected in series to form a single 24V string.

Solar Charge Controller (SCC)
Installed: Apollo T-80
http://www.apollo-solar.net/T80-turbocharger.htm

Product Specifications
Nominal system voltage: 12/24/36/48V
Max module current: 70A
Max charge current: 80A
Overvoltage protection: >140V
MPPT enabled

Charge controllers are the brains of off-grid PV systems. These devices control the flow of current within the PV system to ensure that the electrical power is used or stored in the most efficient manner possible. More sophisticated SCC also have Maximum Power Point Tracking (MPPT) electronics to improve the overall PV output of systems.

------------------ Quick tip------------------

MPPT refers to a feature that allows charge controller to optimize PV output. In most PVs, the maximum power point (MPP) at 25°C is at 16.5 VDC or higher, while a typical battery bank is in the 12 to 15 V range.

This overhead voltage is built into PV modules by their manufacturers to compensate for voltage loss when the modules are hot. Heating a module can cause voltage depression of over 2.5 VDC just from a 25 to 50°C temperature change.The net effect is that PV modules spend most of their lifetime not operating at their MPP.

However, this built-in overhead voltage can actually be harnessed by MPPT electronics. This is done by finding the MPP of the array's IV characteristics, and stepping up the operating voltage so that optimal voltage can be reached. In this way, the PV array is forced to operate at its MPP regardless of module temperature.

This little bit of electronics can enable a PV array to produce about 10 - 30% more output power than it does without MPPT.

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The Apollo T-80 is one of the SCC that would be used in our Solar Lab, it is suitable for PV voltages 12V, 24V or 48V and is able to handle charge currents of up to 70A. It also features MPPT capability.

Close-up of the T-80 display:

SCC are designed to run on battery. This is to ensure that the device remains operational for 24 hours even when the PV modules are not supplying any power:

Like many other newer models of SCC in the market, the T-80 has an in-built memory chip for data storage which allows regular operating data to be recorded. Long-term performance of the PV system can be monitored.

Connection box

Before connecting PV output to the charge controller, we have to protect the system from surge currents or overvoltages.

------------------ Quick tip------------------

Surge currents can occur due to lightning storms or module malfunction in extreme cases. In unprotected systems, the huge currents produced might cause severe damage to the charge controller electronics, batteries and also loads.

Surge Protection Devices (SPDs) are usually used in electrical works to protect systems. These devices are connected in parallel across the isolator switches. When the voltage exceeds a certain level, the SPD will divert the current into a ground-fault return line to prevent damage to equipment.

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With reference to the diagram above, the positive and negative ends of the PV output are connected to the right and left bus bars (hidden behind the blue terminals) respectively. These bars are then connected to the 2-pole isolator switch (white terminal), with surge protection device (black terminal), or SPD for short, connected in parallel.

Our off-grid system involves DC, which requires isolators that have high-speed switching. This is to prevent arcing between the contacts whenever the switch is turned on or off. For our system, we used a DC miniature circuit breaker (MCB) as our switch.

------------------ Quick tip------------------

Unlike AC current, which switches off at every zero-crossing, a DC is constantly at some non-zero value. If two contact points carrying a huge DC is suddenly broken off, a current path between two ends of the switch would perpetuate via an ionized path of air between the contact points. This ionized path is known as an electrical arc.

Electric arcs generate huge amounts of heat which could melt and damage the metal contacts. It may even pose as a fire hazard. Therefore the DC switches have high switching speeds and large separation between contacts to prevent arcing.

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Inverter and load
Installed: Conergy MIC 350 and Pharox LED lamp (4W)
http://www.conergy.de/en/Desktopdefault.aspx/tabid-198/282_read-2361/

Product Specifications
Nominal system voltage: 12V
Continuous output power: 350VA
Max output power: 400VA
Peak output power (for 500ms): 700VA
Power consumption under no load conditions: 9.5W
Maximum input current: 45A

http://www.lemnislighting.nl/pharox_led_bulb.html

Product Specifications
Nominal system voltage: 230Vac, 50Hz
Power consumption: 3.4W
Lifetime: 50,000hrs
Luminosity equivalent: 40W incandescent

A quick setup was created to test our off-grid inverter: a simple load was chosen - Pharox LED lamp - that ran on 220 VAC. The mini-test was successful:

While it may be possible to run DC appliances on off-grid systems, these devices are rare and more expensive. It would make more sense to convert the power from DC to AC form. Hence, we require the use of inverters to provide the required grid voltage of 220V.

Batteries
Installed: HBL NIFE 12V TGI100
http://www.foreenerji.com/pdf/HB-080T_mn.pdf

Product Specifications
Single battery voltage: 12V
Nominal capacity at C10: 100Ah
Battery technology: Valve Regulated Lead-Acid (VRLA), tubular plates
Electrolyte: Gel

Batteries are the lifeblood of off-grid systems, ensuring that the load receives power during non-sunlight hours of the day. In our system, the batteries used are lead-acid GEL 12V batteries (sealed, maintenance-free) and each weighing around 45kg

Two batteries would be wired up in series to form one 24V, 100Ah battery bank.

------------------ Quick tip------------------

The 100Ah refers to the number of ampere-hours that the battery bank can supply if it's discharged at constant current for 10hours. This is the most widely used standard known as C10 capacity. Some battery suppliers, however choose to state the C100 capacity, which usually seems much more

Battery capacity can vary according to the rate at which it is being discharged. A general guideline is the greater the current discharged, the lesser the overall capacity. The chart below features a 600Ah battery, it is a simple illustration of the relationship between capacity and discharge current:

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Unfortunately, there was a mismatch of inverter ratings because the MIC 350 could only run on 12 VDC; not 24 VDC. As such, we had to temporarily connect the inverters to 'one-half' of the battery bank. This method of discharging the battery bank is usually not recommended since batteries in series do not auto-equalise and the unequal loading will cause disparities in the state-of-charge between the two batteries. We would be rectifying this problem by procuring MIC 700 inverters which are rated 24V.

In the meantime, stay tuned for the next installation!