Technical Papers

A next-generation PV plant architecture based on increasing direct current system voltage from 1,000VDC to 1,500VDC holds the promise of a more cost-effective and productive utility-scale plant due to lower installation and maintenance costs. Mahesh Morjaria, Kevin Collins and Michael Stavish of First Solar and Greg Ball of DNV-KEMA Renewables explore some of the challenges associated with the development of the technology and the efforts to address some of those challenges.

PV power plants require proportionally more up-front capital investment to develop and build than their fossil fuel counterparts. Modelling the lifetime performance of a PV power plant is therefore a critical exercise in proving a project’s bankability and securing finance to cover that cost. However, inaccuracies and uncertainties in modelling techniques create risk in the structuring of project finance. Evan Riley of Black & Veatch explores methods for improving the reliability of performance models and how then can be used to demonstrate that a PV facility will meet expectations.

Fixed and tracker solar mounting systems offer various relative cost and performance benefits. But as JA Solar’s Zhang Lan Jun and Gong Tie Yu describe, surprising results from field analysis of different systems used in China hold important lessons for developers wondering which ones to choose.

As PV systems proliferate it is increasingly important to forecast their energy output in order to ensure a safe and reliable integration of their variable output into electric power grids. Dazhi young, André Nobre, Rupesh Baker and Thomas Reindl of SERIS outline a technique for generating large-scale 2D irradiance using data from pyranometers and plane of array cells.

Large-scale PV contractors must perform tests to verify the correct operation of a new installation. Jorge Coello and Leonardo Perez outline the minimum aspects to consider for the commissioning of large-scale PV plants using a methodology that has been successfully implemented in the commissioning of more than 40 PV facilities worldwide.

Global PV end-market demand for PV modules is expected to reach around 50GW in 2014, which has prompted the need for manufacturers to expand capacity to meet demand. With effective module capacity standing at around 45GW at the end of 2013, Photovoltaics International (PVI) has analysed solar cell, c-Si and thin-film capacity expansion announcements that were extensively reported by sister website, PV Tech, from the beginning of 2014 through to the end of November to establish key trends.

The aim of this paper is to dispel the common belief that bifaciality is nonsense as it is not a mature technology, it is expensive and, because in large systems there is limited albedo from the rear side, it only serves the niche market. A complete picture of bifacial cell technologies and module concepts is presented, as well as levelized cost of electricity (LCOE) results for present and future bifacial systems.

In this paper large-area (239cm2) n-type passivated emitter, rear totally diffused (n-PERT) solar cells are compared with state-of-the-art p-type passivated emitter and rear cells (p-PERC) to evaluate potential advantages of n-PERT over p-PERC. In particular, an investigation has been carried out of fully screenprinted bifacial n-PERT solar cells, in which the boron-doped emitter is contacted with aluminiumcontaining silver (AgAl) pastes, as well as of n-PERT back-junction (BJ) solar cells, in which the B-doped emitter is locally contacted with screen-printed Al. Using two separate quartz furnace diffusions for the B- and P-doped regions, efficiencies of up to 20.3% on bifacial n-PERT solar cells and of up to 20.5% on n-PERT BJ solar cells were achieved. In comparison, reference p-PERC solar cells that were processed in parallel achieved efficiencies of up to 20.6% before light-induced degradation (LID), but degraded to 20.1% after 48 hours of illumination. In addition, ion implantation and pre-deposition of dopant sources have been evaluated as alternative technologies for forming the full-area doping of the front and rear wafer surfaces, thus reducing the number of processing steps for n-PERT solar cells. Using ion implantation and a co-annealing step, efficiencies of up to 20.6% for bifacial n-PERT solar cells have been achieved, and of up to 20.5% for n-PERT solar cells, in which the P-doped back-surface field is contacted with evaporated Al. By employing a boron silicate glass (BSG) deposited via plasma-enhanced chemical vapour deposition (PECVD) as a dopant source, along with a co-diffusion step, n-PERT BJ solar cells have been fabricated with up to 19.8% energy conversion efficiency.

A recent revitalization of the passivated emitter and rear cell (PERC) concept in the silicon PV industry has resulted in solar energy conversion efficiencies of greater than 20% being achieved on p-type solargrade single-crystalline silicon (mono-Si) wafers during the past two years or so, thanks to technological advance in the use of aluminium oxide for silicon surface passivation. The research efforts carried out at JA Solar in developing an industry version of PERC cells that can be mass produced utilizing the existing conventional back-surface field (BSF) cell manufacturing platform with moderate retrofitting have yielded 20.5% average conversion efficiency, which can be consistently achieved on p-type Si wafers grown by the Czochralski method. Moreover, the experimental results showed that an average conversion efficiency of 20% is achievable when, in combination with JA Solar’s proprietary light-trapping technique, the same technological approach is applied to the cells using high-quality polycrystalline silicon (multi-Si) wafers produced by the seeded directional solidification method.

During the past two years remarkable performance improvements have been reported for polycrystalline Cu(In,Ga)Se2 (CIGS), CdTe and perovskite thin-film solar cells. In this paper the key breakthroughs in CIGS thin-film technology are reviewed and the scope for further performance improvements by analysing the stillremaining electrical and optical losses in record-efficiency CIGS solar cells is discussed. On the basis of this analysis it is believed that conversion efficiencies up to 25% are achievable with CIGS solar cells in the mid term. Furthermore, the potential for the concept of polycrystalline multi-junction solar cells to push efficiencies even further, towards 30%, is discussed. Finally, a short review of the CIGS market and an outlook from an industrial perspective are presented.