Successful PV installations

The concept of using photovoltaic (PV) systems to generate electrical power from solar energy has held a strong appeal for building services engineers for some years but their widespread use has been restricted by the return on investment. However green a building owner wants to be, there is invariably a payback element to the decision making, and a 20+ year payback will be rejected by the majority of financial directors.

That situation has now changed with the introduction of feed-in tariffs since 1 April 2010. Feed-in tariffs have been specifically introduced to encourage investment in PV systems by rewarding investors and making the financial aspects more palatable. To that end, through the feed-in tariff scheme, energy companies will pay between 29p and 41p per kWh of power generated to the owners of commercial, industrial and residential. This will also apply to other installations where there are no buildings, such as open spaces that can be used to create a large surface area for solar power collection.

To put this into perspective, it has been calculated that a relatively small 25kW installation will return the investment within six years, while larger 100kW+ systems can expect a payback of around eight years. Admittedly these paybacks are still some way off the two to three years preferred by financial people, but there are other factors that will also need to be taken into account when considering overall viability.

One of these, of course, is the Carbon Reduction Commitment Energy Efficiency Scheme (CRCEES), which is set to prove even more expensive for major energy users since its remit changed in the Comprehensive Spending Review. In fact, it is now effectively a carbon tax for those major users and there are strong indications the threshold for participation will soon be lowered to capture more organisations. So using PVs more widely is one way that building operators can reduce their tax burden.

In parallel, there are other environmental imperatives for those organisations that are not in the CRCEES, so it’s a reasonable assumption the building services engineers will be considering PVs for many more projects.

In fact, this increase in demand is already making itself felt and there have already been twice as many installations during 2010 as in 2009, with a significant increase in planning applications for future PV installations. Some landowners are even planning to cover their fields in PVs as it could be more profitable than using it for more traditional purposes. Analysts have predicted that the UK will have around 250MW of installed PV by the end of 2011, compared to 22MW in 2009.

Seeing the full picture

When specifying a PV system it’s important to consider the whole system and be aware of the impact that connections and other components can have on the overall efficiency of the system and the installation phase.

The typical scenario for a PV installation is to connect all of the solar panels in series, taking the combined DC power generated by the array in a string back to an inverter for conversion to AC for use in the user’s premises or export to the national grid. There will also be a disconnection switch for maintenance that needs to be included in the set up. So there are plenty of time-consuming connections to be made and anything that speeds things up will be appreciated by the contractor.

So it makes sense to apply the principles of plug and play wiring that have already proved popular in many other applications. For example, it is now possible to get pre-wired plugs and sockets that will fit the standard connectors supplied with the PV panels.

However, not all plug and play systems are equal and the method of connection is very important. Experience shows that a precision-machined connector with the ability to handle up to 40A will deliver the best connections. The connectors should be designed so it’s easy to ensure all of the strands of the cable are inserted into the connection to maximise performance.

Another consideration here is the material used for the connection as, given the importance of maximising the power generation to get the fastest return on investment, any resistance losses should be minimised. Standard connectors use tin-plated connectors but silver-plated connecters have been shown to reduce losses.

There are also a number of factors that will help to reduce installation time, which will appeal to the installer. A straightforward termination process, for example, will save a lot of time and a crimping approach using a machine contact is proving very popular with contractors. For peace of mind the contact needs to make a snap connection as it slides into the housing, indicating that the connection is made, and it should be easy to remove the contact without the need for specialist tools. This makes it easier and quicker to verify effective contact and to correct errors. The same principles apply to sealing the connector.

Speeding things up

While plug and play connectivity is quite well established on the DC side, in the UK at least, this is not the case on the AC side. Here, electricians are still required to adopt time-consuming hard wiring of connections from the inverter back to the distribution board.

This is where we can perhaps benefit from the UK coming quite late to PVs, as it provides specifiers with an opportunity to learn from the best practice that has been established in the rest of Europe. And it has already been established that the benefits of modular wiring systems described for the DC side can also be reaped on the AC side. If inverters and disconnect switches (and perhaps even the distribution boards themselves) are pre-wired with connectors, the whole electrical connection time can be reduced by up to 20%, depending on the size and configuration of the installation and the site conditions.

In addition there are all the benefits of using pre-tested assemblies, so that on-site problems are avoided and less time is spent in tracing and correcting faults.

It also makes sense to take advantage of the latest technologies that are now coming onto the market. These, themselves, have been developed in the light of experiences in continental Europe and elsewhere and avoid some of the early pitfalls.

All of these factors can be taken into account when specifying systems; not just focusing on the efficiency of the panels themselves but also on the other aspects that will affect performance. These include PV connectors and junction boxes, overvoltage/lightning protection and solar AC distribution units, all of which can be sourced from a single supplier to ensure compatibility and simplify the procurement process.

From experience with other new technologies it’s clear that the success of the early projects will have a strong influence on future uptake and the benefits that accrue. In the case of PVs, building services engineers have an important role to play in ensuring that this technology delivers maximum benefits.

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