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            星火太陽能和你一起了解更多太陽能資訊

            光伏離網系統的設計選型

            返回列表 來源: 古瑞瓦特 發布日期: 2023.01.22 瀏覽次數:
            / 1、離網系統的價值

            通過長期總結來看,離網系統主要基于以下幾個出發點:

            (1)沒有辦法使用市電(電網)的地方 ,如山上、船上、車上、野外等。這是最典型的離網系統需求;80%以上的使用者都是因為不方便用市電,想通過離網系統實現供電。

            (2)有市電、但經常出現不規則停電的地區 。當停電的時候,可通過光伏離網系統繼續供應負載,滿足持續用電需求。

            (3)有市電、但希望通過離網系統降低電費 。離網系統不光可以通過光伏組件來充蓄電池,也可以在費率低的時候通過設置市電給蓄電池充電,然后在費率高的時候通過蓄電池放電來供應負載。

            / 2、典型離網系統組成

            圖片
            離網系統圖

            上圖是一個典型的離網系統的組成圖。需要注意這幾點:

            (1)離網系統指的是系統輸出不需要像并網光伏系統一樣接入市電電網。離網系統的輸出就是供應負載的。

            (2離網系統提供V-F源,它就相當于一個電網的角色。

            (3)離網系統中的主要設備有:光伏組件、離網逆變器、蓄電池組、匯流裝置、配電箱、用電負載。

            (4)最精簡的離網系統可以只包含2個部分:離網逆變器、蓄電池組。

            /3、離網系統設計過程

            (1)確認需求
            做離網系統是個“量體裁衣”的過程,因為多數情況下不同用戶需要帶的負載都一樣,用電時長也不一樣。正確的方式,是先確認需求:即將要帶哪些負載?負載有多大?一天當中使用多長時間?然后推算出需要建多大的系統。例如,華北地區某客戶需求做一套離網電站,主要負載是1臺3500W的鼓風機,每天用電時長3小時,備電需求1天即可。由這些信息可知,負載的功率為3.5KW,1天的耗電量為10.5kWh。

            (2)離網機選型
            早期離網系統控制部分由MPPT控制器+DC/AC轉換器來執行,隨著技術發展,目前主流廠家都提倡使用控制逆變一體機,一臺設備就可同時實現功能,例如古瑞瓦特的SPF ES系列離網逆變器。

            確認好需求后,就來選擇合適的離網逆變器。所選擇的離網機的輸出,必須能覆蓋負載的額定功率,且盡量留一些裕量,為的是系統更可靠。以上圖中的負載為例,額定功率3.5kW,可以選擇SPF 5000 ES離網機,輸出功率為5kW,可以滿足該負載的供電。

            圖片


            在覆蓋負載功率的前提下,需要多少臺逆變器,就要看備電需求了,也就是所謂的“連續陰雨天數”。因為離網系統中的電是組件和離網機合作發的,如果備電需求較大,1臺離網機和它所匹配的組件可能發不出來,所以就需要倍加。

            (3)光伏組件的選擇
            只要是光伏系統,能量來源都是光伏組件(當然,離網系統還有市電作為輸入的情況)。而選定的離網機有其設定的可接受的組件容量,此處可以直接根據離網機的規格參數來配組件。采用目前主流的182組件550Wp*8塊,額定功率4.4kWp,每日平均發電量約16kWh。也可以頂格配甚至超配一點,需要根據具體需求做匹配。

            組件的串并方式其實跟并網逆變器的原理是一樣的,離網機的組串有最大允許接入電壓值,可根據所選組件的參數進行串接。SPF 5000ES離網機的最大PV輸入電壓為450V,MPPT電壓范圍為120V~430V,若選用550Wp組件,一般開路電壓約在49V,可直接按8塊1串,接入離網機的PV輸入端口。

            (4)蓄電池選擇
            蓄電池是離網系統區別于并網系統的一大特點,它是系統的緩沖調節地帶,否則系統會變成一個孤島。

            圖片

            在選擇蓄電池時,還是依據用戶的用電需求來做。以上文舉例來看,客戶有一個10.5kWh的備電需求,就是說,如果沒有光照的情況下,蓄電池需要通過離網機釋放出10.5度電,才能滿足負載供應。

            但是,不論鉛酸類電池還是鋰離子類電池,都有一個放電深度(DoD=Depth of Discharge)的概念或特性,是必須要考慮進去的。就是說,存入蓄電池中的電,不能每次都100%完全釋放完,只能釋放其中的一部分,否則會對蓄電池造成損害致使電池過早失效。

            一般的鉛酸類電池放電深度在60%左右,而鋰電池可以達到90%。在選擇蓄電池組容量時,放電深度也影響到了蓄電池組總的容量大小。還是以上面舉例,選擇鉛酸電池,10.5度電按60%的放電深度來計算,需要配置總共10.5kWh÷0.6=17.5kWh的蓄電池組,這些蓄電池好比要一個容器,將系統所發出的電存儲起來,在負載需要時再放出。

            不需要那么復雜的計算單個蓄電池的容量或規格,5kW離網機要求配置的蓄電池應在200Ah規格以上,知道了這個,只需通過數量調整,配出相同或接近的總蓄電池容量即可。

            1. Value of off-grid systems

            From a long-term summary, off-grid systems are mainly based on the following starting points:

            (1) There is no way to use the electricity (grid), such as mountains, boats, cars, the field and so on. This is the most typical off-grid system requirement; More than 80% of users are not convenient to use the mains, want to achieve power supply through the off-grid system.

            (2) there is mains electricity, but often irregular power outage areas. In case of power failure, the photovoltaic off-grid system can continue to supply the load, to meet the continuous demand.

            (3) Have mains power, but want to reduce the electricity bill through off-grid system. The off-grid system can not only charge the battery by photovoltaic modules, but also charge the battery by setting mains power when the rate is low, and then supply the load by discharging the battery when the rate is high.

            /2. Composition of typical off-grid system

            The picture

            Off-grid system diagram

            The diagram above shows the composition of a typical off-grid system. These points should be noted:

            (1) The off-grid system refers to the system output does not need to be connected to the mains power grid like the grid-connected photovoltaic system. The output of the off-grid system is the supply load.

            (2) The off-grid system provides a V-F source, which acts as a power grid.

            (3) The main equipment in the off-grid system includes: photovoltaic modules, off-grid inverters, battery packs, confluence devices, distribution boxes and electrical loads.

            (4) The most streamlined off-grid system can only contain two parts: off-grid inverter, battery pack.

            /3. Off-grid system design process

            (1) Confirm requirements

            Making an off-grid system is a "discretionary" process, because in most cases different users need to carry the same load, and the duration of electricity is different. The right way is to first identify the requirements: what loads are going to be carried? How big is the load? How long is it used in a day? And then figure out how big the system needs to be. For example, a customer in North China needs to build a set of off-grid power station. The main load is a 3500W blower, which uses electricity for 3 hours every day and can provide backup power for 1 day. Based on this information, the power of the load is 3.5KW and the power consumption is 10.5kWh per day.

            (2) Off-grid machine selection

            Early off-grid system control part by MPPT controller +DC/AC converter to perform, with the development of technology, the current mainstream manufacturers are advocating the use of control inverter integrated machine, one device can achieve functions at the same time, such as Gurewatt's SPF ES series off-grid inverter.

            After confirming the demand, we will choose the appropriate off-grid inverter. The output of the selected off-grid machine must be able to cover the rated power of the load, and try to leave some margin, in order to make the system more reliable. The load in the above figure is used as an example. The rated power is 3.5kW. The SPF 5000 ES off-grid unit can be selected, and the output power is 5kW.

            The picture

            Under the premise of covering the load power, how many inverters are needed depends on the backup power demand, which is the so-called "continuous rainy days". Because the power in the off-grid system is generated jointly by the components and the off-grid machine, if the backup power demand is large, one off-grid machine and its matched components may not be generated, so it needs to be multiplied.

            (3) Selection of photovoltaic modules

            As long as it is a photovoltaic system, the energy source is photovoltaic modules (of course, off-grid systems and mains power as input). The selected off-grid machine has its set acceptable component capacity, here can be directly according to the off-grid machine specifications to match the components. Adopt the current mainstream 182 assembly 550Wp*8 blocks, rated power 4.4kWp, daily average generation of about 16kWh. It can also be matched to the top or even to the top, which needs to be matched according to specific needs.

            The combination of components is actually the same as the principle of grid-connected inverter. The group string of off-grid machine has the maximum allowable access voltage value, which can be connected according to the parameters of the selected components. The maximum PV input voltage of the SPF 5000ES off-grid machine is 450V, and the MPPT voltage ranges from 120V to 430V. If 550Wp components are used, the general open-circuit voltage is about 49V. You can directly press 8 blocks in a series to connect to the PV input port of the off-grid machine.

            (4) Battery selection

            The battery is a major feature of the off-grid system that is different from the grid-connected system. It is the buffer zone of the system, otherwise the system will become an island.

            The picture

            In the choice of battery, or according to the user's demand for electricity to do. For example, the customer has a 10.5kWh backup power requirement, which means that the battery needs to release 10.5kwh of power through the off-grid machine to meet the load when there is no light.

            However, both lead-acid batteries and lithium-ion batteries have a concept or characteristic of Depth of Discharge (DoD=Depth of Discharge), which must be taken into account. That is to say, the electricity stored in the battery cannot be released 100% each time. Only a part of it can be released. Otherwise, the battery will be damaged and the battery will fail prematurely.

            Typical lead-acid batteries discharge to a depth of around 60%, while lithium batteries can discharge to a depth of 90%. When selecting the battery capacity, the discharge depth also affects the total capacity of the battery. Take the above example again, choose the lead-acid battery, 10.5kWh of electricity is calculated according to 60% discharge depth, need to configure a total 10.5kwh / 0.6=17.5kWh battery set, these batteries are like a container, the system to store the electricity, when the load needs to release.

            Do not need so complex to calculate the capacity or specifications of a single battery, 5kW off-grid machine requirements configuration of the battery should be more than 200Ah specifications, know this, just through the number of adjustment, with the same or close to the total battery capacity.

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