resources in the universe is the solar energy. The Sun emits visible light and infrared energy within a specified range of solar radiation spectrum. This radiation spectrum can be used for energy production .
Within this spectrum, the estimated annual potential of solar energy can achieve the 49,837 exajoules (EJ) – enough to supply the entire energy consumption on Earth, not only once but several times .
That is quite a resource that should be used as much as possible, don’t you think?
The most widespread solar technology these days is the Photovoltaics (PV), not only because it has been widely studied since Albert Einstein published a paper called: “The Photoelectric Effect”, but also because the evolution of the digital and electronic era, has allowed us to reduce the costs of manufacturing photovoltaic cells to achievable prices even for residential applications.
But solar energy has much more to offer than just photovoltaics. Currently, there are several technologies that allow us to extract the energy of the Sun – some in the form of electricity and other in the form of heat. Keep on reading to learn about their specs and the latest innovations.
New breakthroughs in solar technology
Now, let us examine the new technology trends the solar industry has come up with recently.
Concentrated Solar Power
Concentrated Solar Power (CSP) is also known as Solar Thermal electricity. This solar technology has been evolving to be used mainly for the industrial or utility purposes. The world’s leading countries in application of this technology are the United States and Spain, where the available CSP capacity accounts for nearly 80 percent of the world’s total CSP capacity .
Concentrated Solar Power is gradually becoming an advantageous alternative to the use of photovoltaics. We can see its growing popularity especially in Spain where the installed capacity on CSP was nearly 2.2 gigawatts (GW) in 2016. In the United States, it was 1.8 GW of available power capacity.
The system uses hundreds of heliostats (mirrors) that focus the sunlight onto a large heat exchanger, known as the receiver. The receiver is located on the top of a tower, which contains a pipe that transports the heat transfer fluid up to the receiver to reach the point where the heat has accumulated. The heat transfer fluid, then, absorbs the heat and flows back to the ground into a thermal energy storage tank.
When the electricity is required, the fluid flows through a pipe side-by-side another separate pipe with water. This pipe goes into the steam generator. Steam generator is a place where water transforms from liquid to gas – steam. The resulting steam is from there directed to the turbine, which generates electricity.
The remaining steam is then condensed and stored in a water tank. The heat transfer fluid is stored in a cool fluid tank that runs back to the receiver in order to repeat the cycle again .
The latest development of CSP has led to the creation of four new types of CSP technologies. They are based on the same principle and share the same components (solar field, power block and storage system), but have different ways of reflecting or concentrating heat. These technologies are:
1. Solar Power Tower (SPT)
This technology offers flexibility in operating temperature up to 565°C (1050°F) and allows to store the heat for up to 15 hours. A particular advantage of this type of solar power scheme is that besides operating with molten salt (one of the best heat transfer fluids available), there are other alternatives like open air or superheated steam, which decrease their operating costs. Although, their application is still in the development phase.
The main disadvantage of this technology is the significant water consumption for cooling process and cleaning of heliostats.
Examples of successful utilization of this technology are the Solar towers of Julich (Germany) and Solugas (Spain). These towers use pressurized air coupled to combined cycle turbines. This solution allows to generate up to 4.6 megawatts from air heated to 800°C (1472°F).
Solar Power Towers are the most used technology of this kind in the United States. The towers are the highest in the world allowing the country to obtain power capacity up to 377 megawatts.
2. Parabolic Trough Collectors (PTC)
The most widespread technology type among CSP and the most commercially mature system as well. Parabolic trough collectors make up 3.5 GW out of the total 4.8 GW of installed CSPs worldwide.
In general, the heat transfer fluid used in this type of installations is the thermal oil that can achieve temperatures between 293°C (560°F) and 393°C (740°F). However, the problem with the thermal oil lies in its high toxicity and flammability. Thus, to lower the risk, latest breakthroughs aim to increase efficiency by using alternative heat transfer fluids like molten salts and air.
It is expected that these alternatives would allow to rise the operating temperature even higher .
3. Parabolic Dish Systems
Parabolic Dish Systems have low application rate in commercial and utility scale projects, although there are still some important projects like Orion project of 60 MW in China.
Despite having higher solar-to-electric conversion than other CSP systems, their downside is the low maximum power capacity of each parabolic dish. This makes them unattractive when compared with other CSP system, especially in terms of costs and power performance. Furthermore, these systems typically lack storage system, making them unviable for large scale utility projects to be connected to the grid.
4. Linear Fresnel Reflectors (LFR)
LFR plants follow the similar trend like parabolic dish systems. Actually, LFR plants do not exceed more than 30 MW and tend to use water as the heat transfer fluid. This generally sets the inlet temperature values between 60°C (140°F) and 190°C (374°F) and exit temperature values between 257°C (495°F) and 370°C (698°F).
Although there are only a few applications in the commercial sector, current trends prove the technical feasibility of this technology, for as long as optical efficiency issues keep improving.
Current development of new projects is located in regions of India, Australia, China, France and South Africa .
Despite that currently the market is ruled by Parabolic Trough Collectors, there is a chance for LFR to be considered a possible competitor of this technology.
Solar Thermal Heating and Cooling
Another type of technology that has been used to extract energy from the Sun are the Solar Heating and Cooling Systems. Their most widely used type are the Solar Water Heating Systems (SWH).
SWHs pass cold water through a pipe that goes into a heat exchanger. From the exchanger, solar collectors carry the selected heat transfer fluid, which is heated from the incoming infrared radiation from the Sun. Once the heat transfer fluid flows back to the heat exchanger through the solar collectors, the stored water is warmed up and then used to heat or cool homes .
According to the REN 21 Global Status Report 2017, nearly 36.7 GW of new solar thermal heating and cooling systems were added in 2016, making it reach 456 GW-thermal worldwide!
This technology is already well-known and can save you money on electricity and gas to provide hot water and heating to your household. Even though the market is already mature, solar thermal heating and cooling is still evolving and new technologies to make these systems even more profitable and efficient keep emerging. The trend has been expanding globally, especially in China, where large multi-home solar heating systems are now more common than traditional heating appliances .
Solar thermal cooling designs are also gaining market shares particularly in warmer regions due to the combination of these technologies with Solar PV, using the electricity generated from photovoltaics to allow a cost-effective operation of compression chillers .
Solar district heating in Denmark
Among recent developments is the innovation of the district solar heating network in Denmark. Denmark has optimized its solar heating system to feed-in solar heat with low feed-line and return temperatures.
The past issues of the system lied in the external regions where the network operated only at higher temperature values. This led to the reduction of conventional solar flat plate collector efficiency. To face this challenge, manufacturers started to develop mid-temperature flat plate collectors that employed either a second glass cover or a foil between the absorber and the glass cover.
This new generation of solar collectors resulted in an outstanding performance!
The Denmark’s solar district heating now get a feed-line temperature between 80 to 129°C (176 to 264°F) and a return line temperature between 58 to 70°C (136.4 to 158°F) .
Improved efficiency of solar collectors
Currently the market offers two main types of solar collectors: unglazed and glazed (flat plate and evacuated tubes).
Glazed solar collectors are the most used worldwide. Particularly the flat plate collectors have a greater usage for their higher efficiency. In laboratory, their efficiency reached even up to 80 percent .
Glazing is a preferred choice because scientists have found out that collector’s efficiency can be further improved by adding transparent insulation materials to the collector’s surface. Glazing performs better simply because this additional layer reduces heat losses between the panel and the environment. This ensures overall better thermal performance of the system .
The newest developments have come up with new designs like thermoplastic natural rubber tubing on absorber plates with foam insulation and water as the working fluid. This combination allows to achieve 72 percent efficiency and reach 65°C (150°F) temperature of the tank. Other remarkable benefits come in the form of low manufacturing costs, high performance and high durability.
Experimenting with different heat transfer fluids
Further developments have been focusing on the heat transfer fluid. Characteristics of this fluid are extremely important, since it absorbs energy in the form of heat from the collector and transmits it to the water in the storage tank.
The properties of the heat transfer fluid like boiling point, freezing point, viscosity and thermal capacity play an crucial role in the overall performance of a solar water system. In general, it is expected that the heat transfer fluid has low freezing point in cold climates and high boiling point in hot climates.
Manufactures are experimenting with different fluids besides water. Some examples are:
- glycol/water mix
- refrigerants (R-11, R-12, R-13, and more)
Ironically, another promising heat transfer fluid is carbon dioxide, mainly because it is non-flammable and non-corrosive.
The main purpose is to apply this fluid to heat the pump of the system .
Latest developments in solar PV technology
The Solar PV market has been expanding exponentially. Costs of silicon solar cells have dropped significantly in the last years, making solar PV systems easily obtainable for many house owners.
Despite the advances, the industry keeps moving forward and new solutions and opportunities appear all the time.
One of the most interesting new trends in the market has been developed by Tesla – the new solar roof!
Tesla, the green tech company, aims to remove any sort of visual setback that homeowners may dislike about solar panels, while at the same time providing a reliable source of PV electricity.
Although, solar roof tiles and building integrated modules have been on the market for some time, the concept of a complete solar roof is new. The full roof implies a solution where PV doesn’t represent an insertion within an already defined roof surface, but instead the solar roof is incorporated into the structure of a house, acting both for aesthetical, constructive and electrical purposes (conceived as solar collectors for energy production) .
Tesla currently offers four different types of solar roofing:
- Tuscan Glass Tiles
- Slate Glass Tiles
- Texture Glass Tiles
- Smooth Glass Tiles
Spray on solar cells
Another solar breakthrough is the Spray On technology. This idea has been circling in the industry for some time since Mitsubishi Corp. decided to invest in the research of this technology.
With the development of a new material – Perovskite (made mostly of carbon and hydrogen) – the final breakthrough happened.
Perovskite allows to harvest light in the form of liquid solar cells. This technology can be placed on diverse surfaces of technically any shape and no furnace is needed. Then, when dried and solidified, perovskite cells can act as semiconductors and generate electricity [14,15].
In contrast with silicon solar cells, the manufacturing and material costs are far more cheaper, as the perovskites are made mainly from carbon .
Spray coating can be also used for fabricating various layers of thin-film solar cells – polymer, quantum dots or colloidal quantum solar cells.
The major limitations for the development of this technology (that is starting to reach efficiency between 10 to 15 percent) are the installation of large-area spray solar cells and the controlled integrity, thickness and internal nano-structure of the thin film.
Hot solar cells
A new interesting approach for obtaining solar electricity has been developed by the MIT researchers in 2017.
Due to the Shockley-Queisser limit , silicon cells are not able to achieve efficiency values beyond 33.7 percent, mainly because the spectrum of solar radiation used to generate electricity is only the visible light. Silicon cells, therefore, can absorb only a fraction of the solar energy.
The MIT scientists decided to change the harvesting procedure. Their idea is that a light concentrator known as absorber-emitter (constituted by carbon nanotubes) should turn sunlight into the heat when temperatures reach 1,000°C (1,832°F). A layer from the emitter, then, radiates the energy back out as light narrowed to the bands that photovoltaics can absorb.
The main benefit of this approach is that it allows to determine which wavelengths of light can flow through the emitter. It also generates more light that the solar cell can absorb, thus, improves significantly solar panel efficiency.
It is still uncertain if this mechanism could be applied on a large scale, as there is only one prototype in the MIT laboratory at the moment. But it is definitely a new breakthrough, worth to be noticed, as the technology addresses changes to the fundamentals of photovoltaic energy conversion [18,19].
Solar panel improvements in efficiency
One of the main factors that influence the output power of a solar PV system is the efficiency of solar panels.
Increasing the efficiency of solar cells has been a major area of investigation in the last 20 years. Selection of materials and manufacturing procedures have allowed the solar industry to present three types of solar panels. Each of them featuring different efficiency levels.
Among the silicon type panels, monocrystalline cells are the most efficient ones, currently reaching levels of efficiency close to 27 percent. On the other hand, polycrystalline panels have evolved quite a lot recently and efficiency levels can reach as high as 22 percent.
Finally, amorphous solar cells should not be left behind and are commercially available with efficiency values between 15 to 22 percent.
Colloidal quantum dots
The key of success is always going forward, and so scientists do not limit themselves to these efficiency values.
If silicon technology cannot achieve higher efficiency values, then maybe light-sensitive nanoparticles might. These so-called colloidal quantum dots are believed to be cost-efficient and flexible materials, ideal for the manufacturing of PV cells.
This technology had already been tested before. The difference now is that this material used previously n-type and p-type semiconductors and unfortunately they were not functioning outdoors.
The new approach allowed for colloidal quantum dots to work outdoors as well, making them suitable for commercial purposes .
Perovskites are probably the major field of study these days to improve solar cells’ efficiency. Perovskites are a new material that be easily used in the large-scale manufacture for its low cost and high efficiency.
Among the interesting properties of perovskites are:
- large spectrum absorption
- long transport distance of electrons and magnetoresistance
There are challenges in this technology regarding the short lifespan, the presence of moisture and the replacement of the lead that they contain into environmentally compatible elements. This requires a deep insight into the perovskites layers.
Although these challenges already seem to be addressed by solar researchers, as particularly in January 2018, scientist from the NREL demonstrated that it was possible to achieve long term stability for perovskite cells by using unencapsulated solar cells that do not have a protective barrier like glass between the cell’s conductive elements .
The solar industry keeps innovating with new technology options that contribute to the process of achieving more effective solar systems and obtaining low cost electricity and heat through the solar energy.
Some of these trends are just in the research phase, other in the development and others are already available on the market.
The important approach for everyone of us is to be updated with the latest trends that will allow you to be one step ahead of the upcoming solar energy revolution.