To achieve wide-scale application without the need to rebuild existing infrastructure it is essential that systems are suitable for installation to allow easy integration with or retrofitting to buildings
Recently, the issues of energy security and an increased desire to curb global warming has led to an increased interest in renewable energy. The EU has aimed to reduce its greenhouse gas emissions by 20% by 2020 and the UK its CO2 emissions by 80% by 2050 compared to 1990 levels. The UK has set a solar power target of 22GW by 2020, India 20GW by 2022 and China 9GW by 2020. Solar energy is recognized to have a high potential as an energy source worldwide as evident from the respective targets set by these countries.
Buildings account for 40% of total EU energy consumption and in the UK buildings are responsible for nearly 50% of total energy consumption. This demand is currently supplied by systems powered by network electricity or fossil fuels, a major source of environmental pollution. Solar energy generation technologies that can produce power with zero carbon emissions and are easily installed in the built environment have a great potential in this scenario.
In this hour of need, the development of a type of solar energy system is required that can be easily integrated into buildings, is compact, lightweight, easy to install and operate, and delivers the maximum power per unit area or per unit installed cost possible. To achieve wide scale application without the need to rebuild existing infrastructure it is essential that systems are suitable for installation onto the existing built environment. The UK has a goal for 2020 of providing 4 million domestic buildings with energy from solar systems.
Concentrating photovoltaic (CPV) systems
Current designs of solar energy systems, concentrating or non-concentrating, have only been partially successful due to their large size, complex operation and high capital cost. Current proprietary PV modules are not able to sustain their growth without government support or subsidies as recently highlighted when PV module sales dropped by almost 90% after the UK government revised the solar feed-in tariff rates in April 2012. If these systems are to be economically cost effective without government support then significant advances in PV cell architecture and the optics of the modules has to be realized leading to a maximum power yield per unit cell area or per unit installed cost. Ordinary PV cells used in conventional flat panel systems deteriorate in performance when exposed to concentrated solar light resulting in their lifespan being reduced by half. Thus, concentrators use specially developed silicon or other material based multi-junction cells. At a concentration ratio of approximately C=42600 multi-stack cells can achieve an efficiency of approximately 90% compared to an average efficiency of 16-17% of commercially available non-concentrating modules.
Triple junction solar cells developed by Sharp have been reported to achieve efficiencies of 35.8% and 42.1% respectively at C=1 and C=230. However, multi-junction cells can be as expensive as £25,000-40,000/m2 compared to typical silicon solar cells costing around £115/m2. Clearly multi-junction cells, despite their high efficiency potential, are expensive and concentrators are essential to make their operation economical.
Concentrators based on heliostat, parabolic trough and linear Fresnel reflector designs are bulkier and complex to track. Low concentration ratio (C≤3) Compound Parabolic Concentrating (CPC) solar collectors offer the most effective opportunity for building integration or retrofit. The rationale for low concentration (C≤3) CPC-based designs is that a fraction of diffused solar radiation can be collected, (1/C). The collection of diffuse radiation in addition to Direct Normal Irradiance (DNI) enables the desired operating temperatures to be achieved at a similar efficiency to systems that concentrate DNI alone; a reduction in the requirement for tracking also leads to a more robust lower cost system.
These systems can be particularly useful in the regions of the world with partly cloudy sky conditions including the UK, mid-northern Europe and India. However, CPC and V trough reflectors or Fresnel lens based concentrators at low concentration ratios (C<10) suffer from non-uniform illumination patterns produced on the cell surface, causing hot spots. This problem needs to be tackled to enhance their suitability for CPV systems. High concentrating systems (C>3) will invariably require some sort of cooling mechanism to lower cell temperatures, with systems with C>30 needing more rigorous cooling scheme.
A renewed focus is required for designing and developing mid-concentration CPV systems (10≤C≤30) that can be installed in existing or new buildings. Such CPV systems may comprise a non-imaging Fresnel lens-based primary concentrator and a CPC or elliptical secondary concentrator. Additionally, these systems can be very useful for integration with cheaper PV cells; due to low concentration they will not cause any excessive heat build-up on the cell array. This allows the cells to operate at lower temperatures, yielding higher conversion efficiencies compared to those with higher concentrating systems. Typically the higher the concentration ratio, the lower the cost of PV systems. However, systems with high concentration ratios (C>30) require two-axis high precision tracking with tolerances below 0.2°.
Another downside is an excessive rise in cell temperature, which can diminish the benefit of augmented solar radiation if not countered by introducing an active method of cooling the PV cell to within permissible temperature limits. Medium concentration CPV systems (10≤C≤30) can be used on buildings roofs or façades due to their capability to be designed with smaller geometric sizes and aesthetic shapes. Due to a comparatively smaller concentration ratio, these will experience a moderate rise in PV cell temperature and can make use of one-axis or less precision two-axis tracking devices. These can have a range of geometric configurations such as parabolic trough, linear Fresnel lenses and reflectors either individually or in combination.
For CPV systems to surpass conventional flat panel systems for building integration or retrofit these must be cost effective, structurally strong, light weight and equipped with a solar tracker of lower complexity. Significant research is clearly required to develop compact CPV modules that have lower capital costs and are easily installed onto new build or existing buildings, domestic, commercial and industrial south-facing structures. Past efforts to realize such technologies have had limited success.
There is a great scope for developing a new CPV system in the mid concentration range (10<C<30) that can maximize the electric power generated using cheaper cells, such as UK-based NAREC’s LBGC PV cells whilst employing a novel concentrator (optical system) that is simple in design, lighter in weight, cheaper to produce and can achieve a higher optical efficiency (≥90%) with less stringent single-axis solar tracking.
Concentrating Solar Thermal (CST) Systems
The industrial and commercial sector accounted for 32% of the final energy consumption in 2008 in the UK and 40% in India in 2010. Industrial heat consumption accounted for 13% of the total UK final energy consumption in 2010 with India consuming 44.8% of its total energy use to supply its industrial thermal energy demand in 2006-07. Most of the industrial heat demand was in the temperature range of 80°C-250°C. The potential for solar collectors operating in the medium temperature range (80°C-250°C), with higher conversion efficiencies than current devices in a cost effective manner, is vast.
Specific technological challenges facing CST systems include their capability to generate heat at >150°C with a minimum 50% collector efficiency and their suitability for retrofitting with existing industrial complexes and buildings. Fixed or biannually tracked CPC solar collectors (C≤3) using a directly absorbing nano-heat transfer fluid (fluid seeded with nano particles to increase its specific heat capacity) circulated through an evacuated all glass absorber tube have the potential to achieve temperatures of 150°C-250°C.
The ability of these systems to concentrate both diffuse and direct normal incident solar radiation is expected to enable the intended temperatures to be achieved in a wider range of locations and the reduction of the tracking requirement will enable fabrication of a more robust lower cost system. The solar-to-thermal conversion efficiency of such systems needs to be experimentally established to increase end-user confidence, to enhance their uptake. Such CST collectors are expected to yield a great ease in installation on current industrial and commercial building structures due to their compact designs and the possibility of integrating light weight polymeric components.
There is a huge potential for developing CSP and CST systems for meeting thermal and electric power demands of new build or existing buildings. A novel approach could be to develop concentrator devices that can be easily integrated to either a thermal absorber or photovoltaic cell array.
This approach will open new opportunities for system developers and manufacturers as the focus will entirely be on designing the concentrating device, which can take either a thermal receiver or photovoltaic receiver as a cassette depending upon the mode in which it is to operate, either heating, power generation or even both, if cooling systems are integrated with CPV modules. This approach could further yield standardized optical concentrators, thermal absorbers and PV receivers for a range of demand sizes and architectural constraints.
However, the challenges that these systems face are those that can’t be tackled by individuals of a singular background or by any one organisation. Rather teams of engineers, physicists, material scientists and economists have to come together to form a coherent team to produce the most appropriate technology. Above all, policy makers have to facilitate this process by creating policy mechanisms which will enable quick and easy knowledge transfer from researchers to industry to minimize the delays in converting ideas into commercial products.