Batteries: Which is best for solar storage?

By Kathie Zipp | August 21, 2015

Interest in energy storage is growing rapidly. It’s not all about living off the grid anymore. Storage helps solve variability issues with renewables. Adding solar batteries to a grid-connected residential project also allows the array to keep providing power to critical loads when the grid is down, instead of having to disconnect and refrain from generating power. Storage can also help commercial consumers reduce peak demand charges, significantly lowering their energy bills. Storage is even used at the utility level to help provide ancillary services to the grid. The need for storage grows as states pass self-consumption legislation.

Batteries in solar applications have to meet the demands of unstable grid energy, heavy cycling (charging and discharging) and irregular full recharging. There’s a variety of battery types fitted for these unique requirements. Considerations for choosing a battery include cost, cycle life and installation and maintenance.

Here’s a look at these aspects of each technology, as well as some best practices when selecting batteries for a solar installation.

Solar battery technologies

Lead acidDeep-cycle, lead-acid batteries have been employed in renewable energy and reliably used in off-grid applications globally for decades.

Cost: Typical deep-cycle, lead-acid batteries cost significantly less than lithium-ion.

Cycling: Valve-regulated lead-acid (VRLA) batteries include absorbed glass mat (AGM) and gel models. Many AGM batteries available in the market are primarily built for dual-purpose or standby applications like emergency backup, but not deep cycling. However, new deep-cycle AGM designs have increased performance and total energy output making them a good choice for renewable energy applications at a lower price point than gel batteries.

In fact, VRLA batteries with added nanocarbon are more resistant to sulfation, which can lead batteries to die over time. The carbon slows sulfation and allows the battery to charge faster and cycle more than traditional lead acid. This makes it a good choice for applications in which the battery is in a partial state of charge, such as energy arbitrage or off-grid.

Replacement/maintenance: Many factors including initial design and ongoing maintenance influence battery life so it’s difficult to put a time frame on when the batteries will need replacement. Flooded lead-acid batteries have to be refilled regularly because the electrolyte that fully submerges the battery plates evaporates during charging. The battery enclosure needs ventilation to keep hydrogen gas from accumulating to dangerous levels.

AGM and gel technologies, however, are recombinant, meaning they internally convert hydrogen and oxygen into water and do not require maintenance. As there is no free acid inside these batteries, they can be installed in any position other than upside down. Because solar applications can be in hard-to-reach or remote areas, the ability to install the batteries and let them operate over long periods without maintenance is a benefit.

Disposal: Proper disposal of lead-acid batteries is important because they are toxic. Thankfully, the automotive industry organized to recycle lead early on. Plastic containers and covers of old batteries can also be neutralized, reground and used in new battery cases. In some cases, the electrolyte is cleaned, reprocessed and sold as battery-grade electrolyte. In other instances, the sulfate content is removed as ammonium sulfate and used in fertilizers. The separators are often used as a fuel source for the recycling process. Old batteries may be returned to the battery retailer, automotive service station, a battery manufacturer or other authorized collection centers for recycling.

Lithium-ionAccording to a U.S. Solar Energy Monitor report, lithium-ion batteries are the most common storage technology, regardless of application. There are three types: pouches such as in smartphones and tablets, cylindrical such as in power tools, and prismatic (which come in various shapes) such as in electronic vehicles. Prismatic types often have corrugated sides, which create air gaps between adjacent cells and can aid in cooling. The prismatic can have applications in solar energy storage, specifically lithium iron phosphate (LFP) batteries.

Cost: Deutsche Bank analysts estimated lithium-ion batteries at about $500/kWh at the end of 2014, but one manufacturer said it’s closer to $750 to $950/kWh. Overall, they are more expensive than lead acid batteries. Part of this cost comes from needing a battery management system to monitor the voltage and temperature of each cell to prevent excessive charging and discharging. A BMS isn’t critical for other technologies like lead acid because the inverter or charger controller can handle the battery charging regime. However, some manufacturers note that, if sized correctly, lithium-ion cells can reduce the cost of peripheral devices like charge controllers, offsetting its higher initial price and lowering cost-of-ownership.

Cycling: Lithium-ion batteries can typically deliver more cycles in their lifetime than lead-acid. This makes them a good choice for applications when batteries are cycled to provide ancillary services to the grid such as energy smoothing or frequency and voltage support. The most important benefit lithium-ion provides for solar is its high charge and discharge efficiencies, which help harvest more energy. Lithium-ion batteries also lose less capacity when idle, which is useful in solar installations where energy is only used occasionally.

Replacement/maintenance: Lithium-ion batteries can be lighter and more self contained than lead-acid batteries, so may be easier to install and change out. They can be wall-mounted and located indoors or outdoors. They are solid, so don’t require refills or maintenance.

Disposal: Lithium-ion batteries can use organic or inorganic cells. Organic-based batteries are free from any toxins. Inorganic-based cells are much more difficult to dispose of. Inorganic lithium-ion is toxic so it must be disposed of properly. Manufacturers encourage recycling, but there is often a price. Spent lithium-ion cells have little commercial value. Lithium-ion manufacturing involves lengthy preparation and purification of the raw material. In recycling, the metal must go through a similar process again, so it’s often cheaper to mine virgin material than retrieve it from recycling.

Flow batteriesRedox flow batteries are emerging as another storage option. Lux Research reports that falling costs will lead to a 360-MWh market in 2020, worth $190 million. The vanadium redox flow battery (VRFB) is the most mature technology in this area.

Cost: VRFB developers say that sourcing vanadium from flyash (a by-product of coal-fired electric generating plants) will reduce costs from over $500/kWh to $300/kWh at scale by 2024. VRFB developers are developing ways to boost power density, which will further drive down costs. Integrated power electronics manage the charging and discharging processes, providing a low cost-of-ownership. But the complexity of flow battery chemistry often requires ancillary equipment such as pumps, sensors, control units and secondary containment vessels. This infrastructure takes up appreciable installation space.  However, one manufacturer has eased the complexity of ancillary equipment by including all required components within the container itself thereby offering a complete built-in solution.

Replacement/maintenance: VRFB manufacturers note the vanadium electrolyte doesn’t degrade over time, so they can last much longer than other technologies. With other technologies, adding more batteries is the only way to increase hours of storage. A benefit of VRFB architecture is that you can increase battery size by simply adding more electrolyte.

Cycling: VRFB developers say the technology has no cycling limitations, and batteries can be charged and discharged completely without impact on their lifespan.

Disposal: The recycled vanadium in flow batteries is not toxic and can be reused repeatedly for other purposes, such as in making steel. Flow batteries contain an aqueous-based electrolyte that can’t get hot or catch fire and thus are intrinsically safe.

Nickel cadmium 
Nickel cadmium or NiCd batteries have been around since the early 1900s. Though they may not have the energy density (the power) of other technologies, they provide long life and reliability without complex management systems.

Cost: Nickel cadmium is relatively inexpensive compared with other technologies.

Replacement/maintenance: NiCd batteries are vented to allow gases to dissipate. They traditionally require some watering, but new designs allow the gases to recombine to form water which makes the battery nearly maintenance free. This, along with the ability to tolerate extreme temperatures, makes these batteries ideal for off-grid applications in harsh environments. They have been used for storage in megawatt-sized projects. .

Cycling: NiCd batteries are rugged batteries with a high cycle life. Some companies promise a service life of up to 20 years.

Disposal: Cadmium is a hazardous material. In fact Europe limits the applications NiCd batteries can be used in. Toxic materials must be removed before the battery is disposed of. NiCd batteries can be recycled, however. The cadmium can be extracted and reused in new batteries. The nickel can be recovered and used to make stainless steel.

Choosing the right battery

Use a sizing calculator
Battery sizing is essential but often overlooked by users and installers. Batteries in PV systems are routinely undersized due to cost or because the system loads were underestimated. It’s important to know the customer’s power needs and correctly plan. Many online calculators provided by battery manufacturers and other software simplifies determining battery capacity for load requirements.

Consider cost of ownership
There are several factors that should be taken into account when determining the total cost of ownership over the life of the battery.
• Price: A battery with a low price is always attractive, but if low price comes at the expense of quality and battery life, the need for frequent battery replacements could boost the cost over time. That’s why it’s important to consider issues other than price when making the decision.
• Capacity: Battery capacity is important because it’s a measure of the amount of energy stored in the battery.
• Voltage: The battery bank voltage must be considered to ensure it matches the system requirements. The battery bank voltage is often determined by the inverter specifications if installing a DC-to-AC system or by the voltage of the loads in a DC system.
• Cycle Life: The most critical consideration is cycle life, which provides the number of discharge/charge cycles the battery can provide before capacity drops to a specified percentage of rated capacity. Batteries from different manufacturers may have the same capacity and energy content and be similar in weight. But design, materials, process and quality influence how long the battery will cycle.

Battery ratings
The nameplate rating on a battery is the fully developed capacity, so it can be misleading to test a battery immediately after it is purchased because it may take up to 100+ cycles for it to reach its full capacity. Beware of batteries that promise full capacity at the time of purchase or those that reach full capacity after only a few cycles. Batteries with a 100+ cycle warm-up will always outlast those touting a high initial capacity.

This article has been updated as of 8/2017.

PWM vs MPPT solar charge Controller

MPPT vs PWM Solar Controllers

Which is the best solar controller for my system?

 MPPT or PWM?

We have compiled a small amount of information that will hopefully help you to
understandbetter what the difference is, and why you should choose an MPPT or
a PWM type Solar Controller for your system.

A solar charge controller is needed in virtually all solar power systems that 
utilise batteries. The job of the solar charge controller is to regulate the power 
going from the solar panels to the batteries. Overcharging batteries will at the 
least significantly reduce battery life and at worst damage the batteries to the 
point that they are unusable.

The most basic charge controller simply monitors the battery voltage and opens
the circuit,stopping the charging, when the battery voltage rises to a certain level.
Older charge controllers used a mechanical relay to open or close the circuit,
 stopping or starting power going to the batteries.

More modern charge controllers use Pulse Width Modulation (PWM) to slowly
 lower the amountof power applied to the batteries as the batteries get closer and
 closer to fully charged. This type of controller allows the batteries to be more fully
charged with less stress on the battery, extending battery life. It can also keep
batteries in a fully charged state (called “float”) indefinitely.
 PWM is more complex, but does not have any mechanical connections to break.

The most recent and best type of solar charge controller is called
Maximum Power Point Tracking or MPPT.
MPPT controllers are basically able to convert excess voltage into amperage.
This has advantages in a couple of different areas.

Most solar power systems use 12 volt batteries, like you find in cars.
(Some use other voltages and the same advantages apply to these systems as well.)
Solar panels can deliver far more voltage than is required to charge the batteries.
By, in essence, converting the excess voltage into amps, the charge voltage
can be kept at an optimal level while the time required to fully charge the batteries
 is reduced. This allows the solar power system to operate optimally at all times.

Another area that is enhanced by an MPPT charge controller is power loss.
Lower voltage in the wires running from the solar panels to the charge controller results
in higher energy loss in the wires than higher voltage. With a PWM charge controller
 used with 12v batteries, the voltage from the solar panel to the charge controller
typically has to be 18v. Using an MPPT controller allows much higher voltages in the
cables from the panels to the solar charge controller. The MPPT controller then converts
the excess voltage into additional amps. By running higher voltage in the cables from the
 solar panels to the charge controller, power loss in the cable is reduced significantly.

When using high voltage “Grid Connect” panels with VOC voltages above 35v to charge a
12v battery bank, the only controller option is an MPPT charge controller.

The final function of modern solar charge controllers is preventing reverse-current flow.
At night, when solar panels are not generating electricity, electricity can actually flow
backwards from the batteries through the solar panels, draining the batteries.
 You’ve worked hard all day using solar power to charge the batteries;
 you don’t want to waste all that power! The charge controller can detect when no
 energy is coming from the solar panels and open the circuit,
disconnecting the solar panels from the batteries and stopping reverse current flow.

When assessing which type of solar charge controller to purchase, you need to know
 about their functionality and features but it’s also helpful to see a straightforward
comparison of your options. To that end, we’ve put together a comprehensive look at the
 pros and cons of both PWM Type Solar
Controllers and MPPT Solar Charge Controllers for your convenience!

PWM Type Solar Controllers	MPPT Solar Controllers
PROS
– PWM controllers are built on a time tested technology. They have been used for years in Solar systems, and are well established – These controllers are inexpensive, usually selling for less than $350 – PWM controllers are available in sizes up to 60 Amps – PWM controllers are durable, most with passive heat sink style cooling – These controllers are available in many sizes for a variety of applications	– MPPT controllers offer a potential  increase in charging  efficiency up to 30% – These controllers also offer  the potential ability to have an  array with higher input voltage  than the battery bank – You can get sizes up to 80 Amps – MPPT controller warranties  are typically longer than PWM  units – MPPT offer great flexibility  for system growth – MPPT is the only way to regulate grid connect modules  for battery charging
CONS
– The Solar input nominal voltage must match the battery bank nominal voltage if you’re going to use PWM – There is no single controller sized over 60 amps DC as of yet – Many smaller PWM controller units are not UL listed – Many smaller PWM controller units come without fittings for conduit – PWM controllers have limited capacity for system growth – Can’t be used on higher voltage grid connect modules	– MPPT controllers are more expensive, sometimes  costing twice as much as a  PWM controller – MPPT units are generally  larger in physical size – Sizing an appropriate Solar array can be challenging  without MPPT  controller manufacturer guides – Using an MPPT controller  forces the Solar array to be  comprised of like photovoltaic  modules in like strings

Solar Charger MPPT

Maximum Power Point Tracking atau sering disingkat dengan MPPT merupakan sebuah sistem elektronik yang dioperasikan pada sebuah panel photovoltaic (PV) sehingga panel photovoltaic bisa menghasilkan power maksimum. Perlu diperhatikan, MPPT bukanlah sebuah sistem tracking mekanik yang digunakan untuk mengubah posisi modul terhadap posisi matahari sehingga mendapatkan energi maksimum matahari. MPPT benar-benar sebuah sistem elektronik yang bisa menelusuri titik power maksimum power yang bisa dikeluarkan oleh sebuah panel PV.

Mengapa MPPT ini sangat penting dan apa yang terjadi apabila MPPT tidak digunakan? mari kita lihat gambar di atas. Bayangkan apabila sebuah PV panel dihubungkan langsung pada sebuah charger/disharger batere. PV panel tersebut mempunyai karakteristik seperti yang ditunjukkan pada gambar/grafik di atas yaitu pada temperatur  dan insolasi . Bisa dilihat pada grafik bahwa apabila MPPT tidak digunakan, maka power yang bisa diekstrak dari PV panel hanyalah 53Watt pada 12Volt atau dengan kata lain power maksimum yang bisa digunakan hanyalah 70.67% dari power maximum sebenarnya. Dengan menggunakan MPPT maka power maksimum yang bisa diambil dari PV panel bisa dicapai.

Seperti apa bentuk MPPT secara fisik?sebuah sistem MPPT di sini merupakan sebuah DC/DC converter dengan sebuah controller. Sebuah DC/DC conveter digunakan pada sistem MPPT seperti pada gambar di atas. Pada contoh di atas, sebuah PV panel mempunyai maximum power 75Watt, tegangan maximum 17Volt dengan arus maximum sekitar 4.4Ampere.  DC/DC converter tersebut akan menkonversi tegangan 17Volt dari PV panel menjadi tegangan batere sebagai output. Arus charge batere adalah menjadi . Tentunya arus tersebut nilainya akan bervariasi tergantung dari penggunaannya itu sendiri

MOSFET vs. IGBT

When it comes to SMPS applications, both transistors have their advantages, but which one’s right for you?

BY JEFFREY BAUSCH

There are many types of switch-mode power supply (SMPS) transistors to choose from today. Two of the more popular versions are the metal-oxide semiconductor field effect transistor (MOSFET) and the insulated-gate bipolar transistor (IGBT). Historically speaking, low-voltage, low-current and high switching frequencies favor MOSFETs. High-voltage, high-current and low switching frequencies, on the other hand, favor IGBTs.

Some additional basics:

While everyone has an opinion on which device works best in an SMPS application, the truth is this: there’s no universal standard to determine which device offers better performance in a specific type of circuit. It varies from application to application, and a wide range of factors, such as speed, size, and cost, all play a role in determining the right choice.

So, rather than say that one is outright better than the other, here’s a basic overview on the differences between both transistors.

The MOSFET in a nutshell

The MOSFET is a three-terminal (gate, drain, and source) fully-controlled switch. The gate/control signal occurs between the gate and source, and its switch terminals are the drain and source. The gate itself is made of metal, separated from the source and drain using a metal oxide. This allows for less power consumption, and makes the transistor a great choice for use as an electronic switch or common-source amplifier.

In order to function properly, MOSFETs have to maintain a positive temperature coefficient. This means there’s little-to-no chance of thermal runaway. On-state losses are lower because the transistor’s on-state-resistance, theoretically speaking, has no limit. Also, because MOSFETs can operate at high frequencies, they can perform fast switching applications with little turn-off losses.

Power MOSFETs

There are many different types of MOSFETs, but the one most comparable to the IGBT is the power MOSFET. It’s specially designed to handle significant power levels. They’re only used in “on” or “off” states, which has resulted in their being the most widely used low-voltage switch. When compared to the IGBT, a power MOSFET has the advantages of higher commutation speed and greater efficiency during operation at low voltages. What’s more, it can sustain a high blocking voltage and maintain a high current. This is because most power MOSFETs structures are vertical (not planar). Its voltage rating is a direct function of the doping and thickness of the N-epitaxial layer, and its current rating is related to the channel’s width (the wider the channel, the higher the current). Due to its efficiency, power MOSFETs are used in power supplies, dc/dc converters, and low-voltage motor controllers.

The IGBT in a nutshell

The IGBT is also a three terminal (gate, collector, and emitter) full-controlled switch. Its gate/control signal takes place between the gate and emitter, and its switch terminals are the drain and emitter.

The IGBT combines the simple gate-drive characteristics found in the MOSFET with the high-current and low-saturation-voltage capability of a bipolar transistor. It does this by using an isolated gate field effect transistor for the control input, and a bipolar power transistor as a switch.

The IGBT is specially designed to turn on and off rapidly. In fact, its pulse repetition frequency actually gets into the ultrasonic range. This unique capability is why IGBTs are often used with amplifiers to synthesize complex waveforms with pulse width modulation and low-pass filters. They’re also used to generate large power pulses in areas like particle and plasma physics, and have established a role in modern appliances like electric cars, trains, variable-speed refrigerators, air conditioners, and more.

Comparing structures

The structures of both transistors are very similar. When it comes to electron current flow, an important difference is the addition of a p-substrate layer beneath the n-substrate layer in the IGBT. In this extra layer, holes are injected into the highly-resistive n-layer, creating a carrier overflow. This increase in conductivity within the n-layer helps reduce the total on-state voltage of the IGBT. Unfortunately, it also blocks reverse current flow. As a result, an additional diode (often referred to as a “freewheeling” diode) gets placed parallel with the IGBT to conduct the current in an opposite direction.

The absence of minority carrier transports allow MOSFETs to switch at higher frequencies. There are, however, two limitations: the transit time of electrons across the drift region and the time required to charge/discharge the input gate and “Miller” capacitances.

Switching power

A reduction in on-state voltage can cost the IGBT to experience slower switching speed at turn-off. The reason is that while electron flow can be abruptly halted simply by reducing the gate-emitter voltage below the gate threshold voltage (as is the case with the MOSFET), there’s still the matter of the holes that are left in the drift and body regions (there’s no terminal connection to remove them). The only way to get them out of there is by sweep-out, which is dependent upon voltage across the device and internal recombination. As a result, the device displays a tail current at turn-off until the recombination is complete. This has always been a big drawback for the IGBT.

Advancements

A lot of these facts cover the historical basics for both devices. Advancements and breakthroughs have led to fairly significant performance improvements throughout the years for both devices.

MOSFETs:

• Improved switching speeds.

• Improved dynamic performance that requires even less power from the driver.

• Lower gate-to-drain feedback capacitance

• Lower thermal impedance which, in turn, has enabled much better power dissipation

• Lower rise and fall times, which has allowed for operation at higher switching frequencies

IGBTs:

• Improved production techniques, which has resulted in a lower cost

• Improved durability to overloads

• Improved parallel current sharing

• Faster and smoother turn-on/-off waveforms

• Lower on-state and switching losses

• Lower thermal impedance

• Lower input capacitance

Conclusion

MOSFETs and IGBTs are fast replacing a large majority of older solid-state and mechanical devices. It’s a movement that doesn’t look like it’s going to slow down any time soon either, especially with the development of silicon carbide (SiC) material quality. SiC power devices are showing developers advantages like less loss, smaller size, and improved efficiency. Innovations like this will continue to push the limits of MOSFETs and IGBTs into higher-voltage and higher-power applications. As a result, tradeoffs and overlaps are likely to continue in many applications. With that being the case, careful analysis of the device itself is perhaps the most logical solution when faced with the task of selecting a transistor for your SMPS application. ■

Related Products: Diodes, Transistors, and Thyristors

Materi Panel Fotovoltaik

Materi Panel Fotovoltaik

Secara umum, sel surya fotovoltaik adalah sebagai berikut :

A Substrat/Metal backing

Substrat adalah material yang menopang seluruh komponen sel surya. Material substrat juga harus mempunyai konduktifitas listrik yang baik karena juga berfungsi sebagai kontak terminal positif sel surya, sehinga umumnya digunakan material metal atau logam seperti aluminium atau molybdenum. Untuk sel surya dye-sensitized (DSSC) dan sel surya organik, substrat juga berfungsi sebagai tempat masuknya cahaya sehingga material yang digunakan yaitu material yang konduktif tapi juga transparan seperti indium tin oxide (ITO) dan flourine doped tin oxide (FTO).

B Material Semikonduktor

Material semikonduktor silikon merupakan bagian inti dari sel surya yang biasanya mempunyai tebal sampai beberapa ratus mikrometer. Material semikonduktor inilah yang berfungsi menyerap cahaya dari sinar matahari. Bagian semikonduktor tersebut terdiri dari dua material semikonduktor yaitu semikonduktor tipe-p dan tipe-n. 

C Kontak Metal/contact Grid

Selain substrat sebagai kontak positif, diatas sebagian material semikonduktor biasanya dilapiskan material metal atau material konduktif transparan sebagai kontak negatif.

D Lapisan Antireflektif

Refleksi cahaya harus diminimalisir agar mengoptimalkan cahaya yang terserap oleh semikonduktor. Oleh karena itu biasanya sel surya dilapisi oleh lapisan anti-refleksi. Material anti-refleksi ini adalah lapisan tipis material dengan besar indeks refraktif optik antara semikonduktor dan udara yang menyebabkan cahaya dibelokkan ke arah semikonduktor sehingga meminimumkan cahaya yang dipantulkan kembali.

E Kaca Pembungkus / Cover Glass

Bagian ini berfungsi sebagai enkapsulasi untuk melindungi modul surya dari hujan atau kotoran.

Prinsip Kerja Solar Modul

Panel fotovoltaik saat ini sudah sangat beragam berkat kerja keras para ilmuwan. Jenis-jenis teknologi sel fotovoltaik pun berkembang dengan berbagai inovasi. Ada yang disebut sel fotovoltaik generasi satu, dua, tiga dan empat, dengan struktur atau bagian-bagian penyusun sel yang berbeda pula. Dibawah, akan dibahas prinsip kerja dan struktur panel fotovoltaik generasi pertama berbasis material silikon.

Sel surya konvensional bekerja menggunakan prinsip aliran elektron positif-negatif (p-n) yang terjadi antara dua material semikonduktor silikon, yaitu antara semikonduktor tipe-p dan tipe-n. Material semikonduktor ini memiliki ikatan-ikatan atom dimana terdapat elektron sebagai penyusun dasar. Struktur aton semikonduktor tipe-n mempunyai kelebihan elektron bermuatan negatif sedangkan semikonduktor tipe-p mempunyai kelebihan lubang (hole) dalam struktur atomnya. 

Silikon murni bersifat sebagai konduktor karena tidak ada satupun elektron yang bergerak bebas, dengan kata lain elektron terkunci dalam struktur kristal silikon murni. Artinya, silikon murni tidak bisa menghasilkan listrik yang maksimal namun mempunyai sifat konduktor yang bagus seperti tembaga. Agar bisa menghasilkan listrik secara maksimal, silikon dimodifikasi dengan menambah unsur lain (doping) menjadi semikonduktor tipe-p dan tipe-n. Silikon di doping dengan phospor untuk menghasilkan semikonduktor-n karena adanya elektron bebas. Atom phospor mempunyai 5 elektron di kulit terluarnya. Untuk semikonduktor-p, silikon didoping dengan boron yang mempunyai 3 elektron pada kulit. Boron banyak memilki lubang bebas karena ketiadaan elektron. 

Proses doping ini membuat silikon modifikasi lebih banyak melepas elektron dan lebih banyak membawa muatan arus listrik dibandingkan silikon murni. Ketika energi diberikan ke kristal silikon (bisa dalam wujud panas), hal ini akan menyebabkan beberapa elektron akan lepas dan meninggalkan atomnya. Setiap elektron akan meninggalkan sebuah hole (lobang) disekitar atom dimana elektron bisa diikat. Elektron ini kemudian lepas secara acak disekitar kisi – kisi dari kristal atom tersebut untuk mencari hole lain yang kosong untuk ditempati. Elektron ini disebut sebagai elektron bebas dan dapat membawa arus listrik

Peran dari semikonduktor p-n adalah untuk membentuk medan listrik sehingga elektron (dan hole) bisa diekstrak oleh material kontak untuk menghasilkan listrik. Ketika semikonduktor tipe-p dan tipe-n terkontak, maka kelebihan elektron akan bergerak dari semikonduktor tipe-n ke tipe-p sehingga membentuk kutub positif pada semikonduktor tipe-n, dan sebaliknya kutub negatif pada semikonduktor tipe-p. Akibat dari aliran elektron dan hole ini maka terbentuk medan listrik yang mana ketika cahaya matahari mengenai susunan semikonduktor p-n ini maka akan mendorong elektron bergerak dari semikonduktor menuju kontak negatif, yang selanjutnya dimanfaatkan sebagai listrik. Sebaliknya, lubang bergerak menuju kontak positif menunggu elektron datang,

Mengenal panel surya Macam macam panel matahari

Energi surya atau energi matahari dapat dimanfaatkan dengan menggunakan alat yang dikenal dengan panel surya. Sebagai alat penangkap energi matahari, panel surya merupakan barang yang wajib dimiliki jika ingin energi surya menjadi bagian dari kehidupan kita. Kreatifitas manusia melalui kegiatan riset yang berkesinambungan telah menghasilkan berbagai jenis panel surya. Sekarang, orang bisa memilih panel surya berdasarkan manfaat yang ingin didapatkannya, teknologi, dan kemampuan finansial yang dimilikinya.

Panel surya dibagi tiga, yaitu panel penangkap panas (solar thermal), panel yang memproduksi listrik (dari aliran elektron akibat sinar matahari yang diterima oleh dua lempeng dioda/efek fotovoltaik), dan panel hibrid yang menggabungkan fungsi panel thermal dan fotovoltaik. Panel penangkap panas merupakan alat yang menyerap panas matahari. Panas yang diserap kemudian dimanfaatkan untuk berbagai kebutuhan, diantaranya untuk kebutuhan air panas (air panas untuk mandi), memanaskan suhu ruangan (untuk negara-negara 4 musim yang mengalami musim dingin/salju), dan menghasilkan uap untuk pembangkit listrik tenaga uap). Menara pembangkit (power tower) adalah jenis pembangkit listrik yang memanfaatkan panel surya penangkap panas. Sementara, panel fotovoltaik hanya untuk pembangkit listrik.

Prinsip kerja panel surya adalah mengubah radiasi sinar matahari menjadi panas atau aliran elektron. Radiasi matahari terdiri dari radiasi gelombang elektromagnetik inframerah (panjang) dan ultraviolet (pendek). Radiasi inilah yang akan ditangkap oleh panel surya untuk diubah menjadi panas atau efek fotovoltaik. 

1. Panel Surya Penangkap Panas (thermal)

Panel surya penangkap panas secara umum diasosiasikan dengan panel untuk memproduksi air panas. Panel penangkap panas ini bisa menghasilkan panas hingga ribuan derajat celsius. Tingginya panas yang bisa dicapai ini membuat energi matahari bisa dimanfaatkan di berbagai bidang seperti, untuk penghangat ruangan (dibutuhkan di negara-negara 4 musim), memasak, produksi air panas untuk rumah tangga dan kegiatan komersial, industri, dan pembangkit listrik berbasis uap. 

Tantangan utama penggunaan matahari sebagai energi adalah ketersediaan sepanjang waktu, baik pada saat medung maupun malam hari. Ide yang muncul adalah menyimpan panas yang dihasilkan pada saat siang hari sehingga bisa dimanfaatkan pada saat mendung atau malam hari. Berbagai cara telah dilakukan oleh para ilmuwan, yang paling populer adalah menyimpan panas dalam cairan dalam wadah yang tertutup (tanki) yang memiliki prinsip seperti termos air panas yang biasa kita jumpai di rumah-rumah untuk membuat kopi. 

Berbagai media telah diuji untuk menyimpan panas matahari dalam tangki, termasuk air, udara, minyak, dan natrium. Dari berbagai media ini, Rockwell International memilih garam cair sebagai media terbaik. Garam cair dipilih karena memiliki beberapa keunggulan, diantaranya cair pada tekanan atmosfer, murah, suhu operasi cocok dengan berbagai aplikasi yang secara umum ada disekitar kita, tidak mudah terbakar, dan tidak beracun. Garam cair juga telah banyak digunakan oleh industri kimia dan logam untuk menyimpan dan mendistribusikan panas. Banyak industri di dunia yang telah memiliki pengalaman dan teknologi pemanfaatan garam cair. Saat ini, telah ditemukan senyawa garam cair yang bisa mencapai suhu 500 derajat celsius. Garam cair bersuhu tinggi ini disimpan di tanki tertutup. Panas yang tersimpan bisa bertahan dan dimanfaatkan dalam 1 minggu. Keunggulan garam cair inilah yang membuat panas radiasi matahari bisa dimanfaatkan saat cuaca berawan dan malam hari. 

Bahkan, sistem penyimpan panas menggunakan garam cair telah diaplikasikan dalam pembangkit listrik. Untuk pembangkit 100 megawat, peneliti menggunakan tangki berukuran tinggi 9 meter dan diameter 24 meter. Pembangkit listrik Andasol di Spanyol adalah salah satu contoh penggunaan garam cair untuk penyimpanan panas. Pembangkit ini telah beroperasi pada Maret 2009 dan berhasil menghasilkan listrik 24 jam penuh (siang dan malam). 

Penel surya penangkap panas dibagi tiga, yaitu pelat datar, tabung, dan cekung.

A Panel Surya Pelat Datar

Panel surya pelat dasar dikembangkan oleh Hottel dan Whillier pada 1950-an. Panel ini terdiri dari (1) pelat datar berwarna gelap yang menyerap panas, (2) penutup transparan yang berfungsi mengurangi kehilangan panas, (3) penyalur panas (bisa berupa udara, antibeku, atau air) untuk menyerap panas dari pelat datar, dan (4) isolasi panas pendukung. Pelat penyerap panas terbuat dari lembaran pelat tipis (polimer termal stabil, aluminium, baja atau tembaga). Agar efektif menyerap panas, pelat diberi lapisan hitam (bisa berupa cat). Pelat dibungkus dengan isolasi dengan kaca atau polikarbonat bening, dimana terdapat ruang yang dapat dialiri cairan untuk memindahkan panas panas dari pelat. Cairan panas inilah yang akan dimanfaatkan sebagai pemanas melalui transfer panas langsung maupun tak langsung.  Contoh gambarnya bisa dilihat disini.www.visualdictionaryonline.com, diakses 2016.

B Panel Surya Tabung

Panel matahari tabung lebih jamak digunakan oleh masyarakat Eropa. Negara 4 musim ini memanfaatkan tabung agar penyerapan sinar matahari lebih efektif sepanjang tahun tanpa terganggu dengan musim. Prinsip kerjanya sama dengan pelat datar. Bedanya, pada panel matahari tabung pelat ditempatkan di tengah-tengah tabung kaca. Bersama dengan pelat ditempatkan cairan atau udara.  Cairan dalam tabung (bisa berupa air atau cairan anti beku) akan meningkat suhunya akibat radiasi matahari. Panas cairan ini akan dialirkan untuk pemanas baik secara langsung maupun tak langsung. Dalam satu panel surnya, dipasang beberapa tabung (pipa) penyerap panas.  Contoh gambarnya bisa dilihat di www.alternativeheatinginfo.com.

C Panel Surya Cekung

Panel matahari cekung bisa berupa parabola dan bentuk cekung. Bentuk cekung ini membuat radiasi matahari bisa dikonsentrasikan untuk menghasilkan suhu yang tinggi. Pada panel pelat datar dan tabung, suhu yang bisa dicapai berkisar 95-200 derajat celsius. Sementara untuk cekung bisa mencapai hingga ribuan derajat. Rekor suhu 3.500 derajat celsius dicapai oleh peneliti di Prancis dengan panel berbentuk parabola. Contoh gambar nya bisa dilihat dihttp://www.seia.org/policy/solar-technology.

2. Panel Surya Fotovoltaik

Panel fotovoltaik secara umum terdiri dari dua material semikonduktor yang terhubung, dimana jika terkena cahaya matahari dapat menghasilkan listrik. Pada kondisi gelap atau tidak cukup cahaya berfungsi seperti dioda. Beberapa material semikonduktor yang digunakan diantaranya, monocrystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride, and copper indium gallium selenide/sulfide.

Ketika terkena sinar matahari, umumnya satu sel surya menghasilkan tegangan DC sebesar 0,5 sampai 1 volt dan arus dalam skala milliampere per cm persegi. Agar sesuai dengan berbagai alat elektronik material semikonduktor disusun secara seri dan pararel untuk menghasilkan tegangan DC sebesar 12 V dan arus listrik yang lebih besar. Satu panel surya biasanya terdiri dari 28-36 sel surya (Air Mass 1.5).