How To Choose Solar Storage Battery

The difference between off-grid and grid-connected

Grid-connected: A power generation system that must be connected to the public grid, that is, solar power, home grids, and public grids are linked together and must rely on the existing grid to operate. The grid-connected photovoltaic power station has no electric energy storage device, and is directly converted into the voltage required by the national grid through the inverter.

Off-grid: Also known as an independent photovoltaic power station, a power generation system that operates independently of the grid. The electricity emitted by the solar panel directly flows into the battery and is stored. When it is necessary to supply power to the electrical appliance, the DC current in the battery is converted into 220V AC by the inverter. This is a repetitive cycle of charging and discharging. The system must be equipped with a battery.

Why choose an off-grid power supply system?

The off-grid power supply system is specially designed for use in areas without power grids or places with frequent power outages. It is a rigid demand. It relies on the working modes of "storage and use" and "first-storage and then-use" to provide users with help and services that "deliver charcoal in the snow".

The system operates independently without relying on the power grid and is not subject to geographical restrictions. It can be installed and used as long as it is exposed to sunlight. It is widely used in remote areas without power grids, isolated islands, fishing boats, communication base stations, street lights and outdoor breeding bases. Emergency power generation equipment in power outage areas.

01 Off-grid system energy storage battery

The role of energy storage batteries

In the photovoltaic off-grid system, the energy storage battery occupies the main part, and its main task is to store energy, ensure the stability of the system power, and ensure the load power consumption at night or in rainy days.

Energy storage function: The photovoltaic power generation time and the load power consumption time are not necessarily synchronized. The photovoltaic off-grid system can generate electricity only when there is sunlight. The power generation reaches the highest at noon, but the electricity demand is not high at noon. Many household off-grid power stations are in Electricity only at night. Therefore, the electricity generated during the day needs to be stored by the battery first, and then the electricity is released after the peak of electricity consumption.

Stable system power: Photovoltaic power and load power are not necessarily the same. Photovoltaic power generation is affected by radiation and is in a fluctuating state, and the load side is not very stable. The starting power is greater than the daily operating power of the load end. If the photovoltaic power generation end is directly connected to the load, it is easy to cause the system to be unstable and the voltage to fluctuate. The energy storage battery is a power balance device at this time. When the photovoltaic power is greater than the load power, the controller sends the excess energy to the battery pack for storage. When the photovoltaic power cannot meet the load demand, the controller sends the battery power to the battery. towards the load.

Types and characteristics of energy storage batteries

The energy storage battery is indispensable in the off-grid system. The solar panel charges the energy storage battery through the controller, and the energy storage battery inverts the output for the user through the off-grid inverter. The energy storage battery is compared in the photovoltaic off-grid system. Commonly used are lead-acid gel batteries, ternary lithium batteries and lithium iron phosphate batteries.

Lithium iron phosphate battery is a battery that integrates many advantages. It has high specific energy, small size, fast charging, long service life and good stability.

Its deep cycle charging times are generally 1500-2000 times. A set of lithium iron phosphate batteries can be used in an environment of -40°C~70°C under normal use. Auxiliary heating devices are recommended in extremely cold environments. Lithium iron phosphate battery is an energy storage battery developed in China with mature and independent property rights. The conditions of use and safety protection are well done.

Lead-acid colloidal battery is equivalent to an upgraded version of lead-acid battery. It is maintenance-free and solves the criticism of frequent maintenance of lead-acid batteries. The internal colloidal electrolyte replaces the sulfuric acid electrolyte. improved.

The gel battery can be used in the temperature range of -40℃~65℃, with good low temperature performance, can be used in the northern alpine region; good shock resistance, can be used safely in various harsh conditions; service life is the same as that of ordinary lead-acid batteries double or even more.

High energy density: The energy density of ternary materials is more than 200Wh/kg, which is 3 times that of nickel-metal hydride batteries and 5 times that of lead-acid batteries.

Wide operating temperature range: The battery can be used in the environment of -20℃~60℃, it can reach 80% under 1C discharge at -20℃, and it can reach 70% at -40℃.

Long cycle life: single cell cycle life with high energy density > 1500 times, power type can reach more than 2500 times (capacity retention rate > 80%) 

How About Cover A City With Solar Cells

 Now that the weather continues to be hot and hot, some people have a sudden idea: Taking advantage of the east wind of the photovoltaic industry, if we directly cover the city with solar panels, it will not only solve the problem of electricity consumption, but also not get hot, is this kind of project feasible?

1. China also has this idea at the national level, and is developing the whole county of photovoltaics in an orderly manner

One of my classmates is in a central power company and is currently doing photovoltaic development. He said this, if they can outsource the right to develop photovoltaics in a city, they can install photovoltaic panels everywhere. And the country also has this idea, so it is successively engaged in photovoltaic development projects in the whole county.

In other words, this is already being done, but the cost of urban transformation is relatively high, so we can only start from the towns and counties with better lighting.

For example, Gonghe County, Qinghai Province, its photovoltaic panels cover more than 609 square kilometers, which is almost the size of Singapore.

According to reports, the average annual power generation of the photovoltaic industry in Qinghai Province is as high as 80 million kilowatt-hours. In addition to meeting the consumption in the province, the power supply of these clean energy can also be delivered to Jiangsu, Henan and other places, and also drive nearly 1.2 million public welfare poverty alleviation jobs. post.

It can be seen that this idea is very interesting. For the sake of sacrifice, I dare to call the sun and the moon a new sky. Of course, we also need to know that the current solar panels cannot prevent the sun from being heated, and the power generation efficiency is about 20%, and 80% of the sunlight energy will still be converted into heat energy. Therefore, even if the city is fully covered, the temperature will not change much from normal.

However, as long as 20% of the heat energy used in the city is converted, it is equivalent to 20% less electricity input to the city from the outside. So, is full coverage of solar cells in cities feasible?

2. Do the math: How high is the input cost if the city is covered with solar cells to generate electricity?

Take the city of Shanghai as an example: it costs about 100 to 120 yuan per square meter to mount a glass panel or solar panel to a bracket about 6 meters high on the flat ground. In the city, the total cost will increase if the support has to adapt to the existing buildings and terrain fluctuations, cope with the impact of terrain obstacles and various weathers, occupy part of the built-up area, and acquire land if necessary.

In addition, the cost of solar panels is about 270 to 740 yuan per square meter, and at least about 40 yuan per square meter is required for maintenance each year. Moreover, the urban area of ​​Shanghai's central area is about 664 million square meters, and there are a lot of building height differences. Covering this area requires the cost of brackets and solar panels to exceed 1 trillion yuan. Do you think this cost is acceptable?

At the same time, solar panels will not completely prevent the heating caused by sunlight, and the urban air flow, lighting, rainwater treatment and other aspects under the coverage need additional design, as well as land acquisition, procurement, construction, etc., and the cost of supporting equipment is at least hundreds of billions of yuan.

Let's look at Shanghai's GDP of 3.87 trillion yuan in 2020, general public budget revenue of 704.63 billion yuan, and general public budget expenditure of 810.21 billion yuan. Shanghai has so much money to spend to build such a project?

And every year, these solar panels require tens of billions of dollars in maintenance costs, and each solar panel needs to be replaced and scrapped after about 25 to 30 years, which is no less expensive than rebuilding.

Under the condition of Shanghai's annual average illumination, the above-mentioned solar panels can generate about 110 billion kWh of electricity each year, and the power fluctuates greatly with time and weather. In 2020, Shanghai's electricity consumption is about 157.6 billion kWh. That said, the aforementioned solar panels are not enough, even if they cost a fortune.

To sum up, even without considering the technical details, the above-mentioned projects are unprofitable at the current productivity level, and it is natural that no one is going to engage in urban full-coverage projects.

3. The types of projects covered by solar panels have indeed appeared in cities.

This can actually be discussed, and it may not be practical to cover it completely, but it is reasonable to install it on the roof, canopy, etc. It can also add some solar awnings to the sidewalks with high traffic to facilitate travel in summer and rainy days.

It can be seen that in the current city, the types of projects covered by solar panels have indeed begun to appear. And they all have one thing in common, in addition to generating electricity, there are other benefits.

One is the closed awning on the roof terrace. Originally it was not allowed to build, but in the name of solar energy, it was allowed to build. There is a lot of extra use area, which is old and valuable, so it doesn't care about the investment income ratio of solar energy itself.

Second, the roof of the villa is also a photovoltaic power generation resource. Only 24,000 yuan can be installed to install 7,000 watts of photovoltaic panels. In the southern region, 7,000 kWh of electricity can be obtained every year. According to the annual per capita electricity consumption of more than 800 kWh in China, it is enough for a family of three.

The third is the solar awning of the parking lot. Originally, the parking lot had to build awnings and charging piles. By the way, the solar awning can be built to generate electricity and supplement electricity, and the benefits will come out.

Fourth, street lamps, decorative lanterns. In some places, it is very troublesome to pull the wires. Using a solar panel to supply energy to the street lights saves a lot of trouble, and the benefits can be shown.

In short, with the improvement of solar panel efficiency, cost reduction, and precise policy guidance, the coverage area of ​​solar panels in cities will inevitably increase, but this must also be a gradual process.

Final words: Urban photovoltaics have a large space for imagination, and technology is developing. We must make use of our strengths and avoid our weaknesses, and we must not give up food because of choking.

We need to know that in many countries, the difference between the use of the public grid and the on-grid electricity price may be as high as 1RMB/kWh. As the price of lithium batteries decreases, for example, Volkswagen expects that the price of lithium batteries will drop to $50/kWh in 2030, even if it is completely dependent on photovoltaics and battery energy storage, it may be more cost-effective than the public grid.

Of course, China has a vast territory and uneven distribution of sunlight resources. Most of the major cities are areas with Class II and Class III sunlight resources. On the contrary, the northwestern desert and other regions have more than 2,000 hours of sunshine throughout the year, which belongs to the first-class light resource area. According to preliminary calculations, only the Taklimakan Desert is fully equipped with photovoltaic panels, which can be more than enough to supply the world's electricity demand. Easy first and then difficult, urban photovoltaic imagination is also very large, let us wait and see.