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You have to admire Akeler. When other commercial office developers have an attack of the vapours when someone suggests something other than a fan-coil, Akeler have no such qualms. Not only was Akeler's speculative Solar Office truly pioneering, two years on it's still unique.

That said, the convincing case for cladding a building with 640 m² of photovoltaics was always going to require the test of time. For two years the project's lead designer, Studio E Architects, has been monitoring the effectiveness of the integrated array₁.

Studio E studied the system's performance to determine whether Akeler's confidence in the technology was justified. Monitoring of the electrical output from the array was carried out over a two year period from April 1998 to March 2000.

PV design details
Maximising power generation from the pvs was always going to be critical, given the conflict between the opaque pv cells and the need to ensure adequate daylight into the offices. It was felt that a facade that was around 40% clear glazing and 60% pv might achieve both ends.

A scale model was made and tested under a range of conditions. This resulted in a design that comprised alternating bands of clear glazing and pv modules, with semi-transparent modules set immediately above the clear glazing. This was in order to avoid a strong contrast between light and dark at around eye level.

The size of the pv installation was a balance of maximising the power output, the size of array that could be integrated into the building and not least the availability of development grants (see box story on page 28).

Electrical performance
In the last two years the pv array has generated some 113 000 kWh/y, which compares well to the design prediction. Most parameters – irradiation, ambient temperature, input and output power – were recorded as average values over 10 minutes. Data were downloaded from an on-site computer on a weekly basis and analysed by Newcastle Photovoltaics Applications Centre.

A total output of 94 790 kWh was recorded between March 1998 and May 2000. Some inverter problems experienced during the summer months masked the seasonal differences in performance that were expected, although there was a noticeable improvement in efficiency during the winter months.

Efficiencies for the four pv sub-systems have also been calculated. These consist of two large arrays connected to 3-phase inverters, and two small arrays connected to single-phase inverters.

The smaller subsystems exhibited consistently lower performance than the larger systems. Half the difference was assigned to the fact that the smaller subsystem array has a higher proportion of lower efficiency modules due to higher cell spacing. The remainder was assigned to a lower efficiency of dc/ac conversion.

It is usual to derate inverters for climates where there is a significant percentage of operation under low light conditions. In this way the loss of output under occasional high insolation conditions is more than offset by the increased efficiency at low insolation conditions.

At Doxford, the large inverters are rated at 98% of the corresponding pv array capacity, while the small inverters are rated at 90% of their corresponding pv capacity.

Assuming similar low level light, the small inverters should give slightly higher subsystem efficiencies. However, the single-phase inverters have exhibited slightly different performances. This is partly due to different solar input thresholds, and partly due to transparency differences between the modules.

Not only was Akeler’s speculative solar office pioneering, two years on it’s still unique

While the major difference between the two inverter sizes is due to low light-level performance, this has had very little effect on total performance as the subsystems with the small inverters represent a small proportion (2.6%) of the array capacity. A much larger loss would have occurred had the entire system been configured of 1 kW strings (figure 1).

System losses
Overall, the pv array efficiency at Doxford is calculated to be 11.3%, based on the watt peak ratings of the individual modules and taking into account their design levels of transparency.

With system efficiency at 8%, about 30% of the expected dc power generation must be due to system losses. Other losses are:

• inverter efficiency
• insolation below inverter thresholds
• module temperature
• soiling (dependent on cleaning; 3% assumed at Doxford)
• angle of incidence (assumed 5% at Doxford)
• performance mismatch between modules and cables (possibly up to 12% at Doxford)

System reliability
Inverters were the main cause of operational problems at the Solar Office, causing intermittent output. These were resolved and the system has since been operating without further problems. The only other reliability problems were caused by regular crashes of the monitoring computer, variously ascribed to corrupted software and clashes with the website software. Other than that, the system is said to have operated with excellent reliability.

Quality standards for the pv modules were another vexed issue and took time to resolve. These included the concealment of circuitry tape at module edges, cell alignment, entrapped air bubbles and tears in the cells.

In the absence of any standard for cell alignment and entrapment of air in the pv resin, the designer and contractor devised an ad hoc standard. With respect to the inadvertent tearing of cells during panel installation, again there is no established standard. The three cells that were found to be torn suffered no loss of output.

One issue which almost derailed the contract negotiations was the warranty period the contractor was offering the developer. Different parts of the pv system came with different warranties – the curtain wall 12 y and the pv modules 10 y. When Schüco was pushed to extend the warranty on the modules, issues of responsibility promptly arose, as well as what would constitute a failure.

Comfort conditions
The building's (partial) occupation by Domainnames.com late in the monitoring programme means that a full user survey of the building and the relationship to the pv system has yet to be carried out. BRECSU has been granted funding to carry out an analysis of the building under their Best Practice Programme, but this is conditional on the building running in a passive solar or mixed-mode regime.

Due to the tenant's high incidental cooling loads, a variable air volume system with mechanical cooling has been installed. It is hoped that this can be modified to run in a mixed-mode format. If so, monitoring and assessment will start in early 2001.

In 1998, there was no UK manufacturer of pvs capable of responding to the requirements of this particular project. While German companies were more able to deliver, the procurement process was hampered by an extended and therefore relatively unresponsive chain of manufacturers, suppliers and specialist installers.

Experience at the solar office also reveals the importance of harmonising the warranties offered by pv manufacturers and installers with those expected of the commercial property market. There are also no readily available models for preparing performance specifications.

The solar facade package was the key to weather-sealing the building. Internal trades could not proceed until it was installed. Hence any slippage in delivery of the components could have compromised the fit-out had it followed on from the main contract.

Akeler took an interest in ensuring that the building's occupiers thoroughly understood its design principles and how to get the best out of it. For this reason Akeler funded a touch screen graphic display located in the atrium.