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1、基于MPPT的智能太阳能充电系统研究 作者：卢琳殳国华等1引言太阳能是一种清洁高效的可再生能源。在阳光充足的白天，屋顶的光伏电池将太阳能转化成电能，供人们在夜晚使用。据专家预测，到2040年，全球的光伏发电量将占世界总发电量的26，2050年后将成为世界能源的支柱。2最大功率点跟踪最大功点跟踪(MaximumPowerPointTracking，简称MPPT)系统是一种通过调节电气模块的工作状态，使光伏板能够输出更多电能的电气系统。图1示出光伏电池输出功率Pb与输出电压ub和输出电流ib的关系。图中A为普通控制器使光伏电池工作在12V，仅输出53W时的功率点(一般功率点)；B为MPPT控制器使
2、光伏电池始终工作在最大功率点，从而输出高达75W时的功率点(最大功率点)。最大功率点主要受环境温度和太阳光强的影响。在太阳光强不变的情况下，随着温度的升高，光伏电池的开路电压降低，最大输出功率随之降低。当温度不变，太阳光强增加时，光伏电池的开路电压基本不变短路电流大幅增加，最大输出功率大幅增加。图2示出线性系统电路图。首先，计算消耗在R1上的功率为：然后，式(1)两边对R1求导可得：由式(2)可得，当r=R1时，dP1dR1=0，此时P1取最大值。由于光伏电池系统受到光强、温度、太阳光入射角等多种因素的影响，其输出电压ub、输出电流ib和内阻r也处于不停变化之中。只有使用DCDC变换器实现负载
3、的动态变化，才能保证光伏电池始终输出最大功率。MPPT需要及时准确地采样蓄电池当前的充电电压和充电电流。两者相乘得到当前的充电功率，与前一时刻的充电功率相比较，调节PWM的占空比，从而使光伏电池始终工作在最大功率点。图3示出具体的控制策略。3MPPT的硬件设计由于光伏电池的输出特性呈非线性，且变化幅度较大，所以使用单端反激式变换器。该变换器由升降压变换器加隔离变压器推演而来，能够简单高效地提供直流输出，广泛用于功率100W左右的小型开关电源中。控制器工作于电流断续模式。图4示出MPPT的硬件设计原理。其中，微控制器采用MC68HC08SR12微处理器，使用AD模块采样电源的输出电流和输出电压，
4、继而调节PWM占空比，最终实现光伏电池的最大功率输出。MPPT控制策略的效果好坏直接取决于电压和电流的采样是否精确。图5示出电压采样电路。它采用光耦PC817和三端稳压管TLA31相配合。TLA3l是一种可编程稳压管，当变压器的次级输出电压uout变化时，光耦的输出电压随之变化，AD会采样到当前的充电电压图6示出电流采样电路。由它对采样电阻Rsam两端的电压进行采样，并使用差分式运算放大器放大输出到MCU的AD采样端，从而得到主电路中的电流值。由于信号需要精确采样，并且与电源隔离，因此使用线性光耦HCNR200。另外，单片机及周边电路的用电可直接通过蓄电池隔离变压得到，系统无须外部电源供电，十
5、分方便。4软件分析由于太阳光强和环境温度的变化是一个缓慢的过程，故参数采样无须高实时性，每隔几秒钟采样一次即可满足要求。产生中断的时间间隔是可以调整的，初期较短，可以迅速逼近最大功率点；后期较长，防止系统在最大功率点附近振荡。为防止系统误判断，每次控制比较，均进行3次，当3次的结果一致时，才实施相应的控制策略，否则重新采样比较，这样便最大限度地保证了系统的正常运行。图7示出实现MPPT的软件流程。由于单片机与开关电源一起工作，相互间的电磁干扰较大，而AD采样要求精确。故需要使用软件数字滤波。这里采用均值滤波，即通过多次采样求平均值的方法，以达到去除干扰的目的。蓄电池采用循环充电方式。以12V蓄
6、电池为例，在充电电压达到14.7V最高限制电压后保持该电压继续充24小时达到饱和。最大充电电流不允许超过额定容量的25(如容量为100A时的蓄电池，最大充电电流为25A)。采用保险丝实现硬件电路的过流保护。图8示出软件保护示意图。5实验结果与展望表1给出了系统的主要组成部分及性能指标。图9反映了2005年12月20日早上9点到下午4点的功率P，MPPT充电器的充电情况及最大功率点的变化情况。12月20日的天气变化较快，时而有阳光，时而多云，时而阴沉。最高MPPT可达到50W，最小只有5W左右，这反映出太阳能电源输出功率的多变性。经测试，该MPPT充电系统反映出了太阳能电源输出功率的多变性，并可
7、以快速跟踪最大功率点，这对光伏系统的稳定高效工作起到了至关重要的作用。努力提高系统的效率和可靠性，进一步改进MPPT算法，则是下一步研究的重点。Editors note: Judson Jones is a meteorologist, journalist and photographer. He has freelanced with CNN for four years, covering severe weather from tornadoes to typhoons. Follow him on Twitter: jnjonesjr (CNN) - I will always wo
8、nder what it was like to huddle around a shortwave radio and through the crackling static from space hear the faint beeps of the worlds first satellite - Sputnik. I also missed watching Neil Armstrong step foot on the moon and the first space shuttle take off for the stars. Those events were way bef
9、ore my time.As a kid, I was fascinated with what goes on in the sky, and when NASA pulled the plug on the shuttle program I was heartbroken. Yet the privatized space race has renewed my childhood dreams to reach for the stars.As a meteorologist, Ive still seen many important weather and space events
10、, but right now, if you were sitting next to me, youd hear my foot tapping rapidly under my desk. Im anxious for the next one: a space capsule hanging from a crane in the New Mexico desert.Its like the set for a George Lucas movie floating to the edge of space.You and I will have the chance to watch
11、 a man take a leap into an unimaginable free fall from the edge of space - live.The (lack of) air up there Watch man jump from 96,000 feet Tuesday, I sat at work glued to the live stream of the Red Bull Stratos Mission. I watched the balloons positioned at different altitudes in the sky to test the
12、winds, knowing that if they would just line up in a vertical straight line we would be go for launch.I feel this mission was created for me because I am also a journalist and a photographer, but above all I live for taking a leap of faith - the feeling of pushing the envelope into uncharted territor
13、y.The guy who is going to do this, Felix Baumgartner, must have that same feeling, at a level I will never reach. However, it did not stop me from feeling his pain when a gust of swirling wind kicked up and twisted the partially filled balloon that would take him to the upper end of our atmosphere.
14、As soon as the 40-acre balloon, with skin no thicker than a dry cleaning bag, scraped the ground I knew it was over.How claustrophobia almost grounded supersonic skydiverWith each twist, you could see the wrinkles of disappointment on the face of the current record holder and capcom (capsule communi
15、cations), Col. Joe Kittinger. He hung his head low in mission control as he told Baumgartner the disappointing news: Mission aborted.The supersonic descent could happen as early as Sunday.The weather plays an important role in this mission. Starting at the ground, conditions have to be very calm - w
16、inds less than 2 mph, with no precipitation or humidity and limited cloud cover. The balloon, with capsule attached, will move through the lower level of the atmosphere (the troposphere) where our day-to-day weather lives. It will climb higher than the tip of Mount Everest (5.5 miles/8.85 kilometers
17、), drifting even higher than the cruising altitude of commercial airliners (5.6 miles/9.17 kilometers) and into the stratosphere. As he crosses the boundary layer (called the tropopause), he can expect a lot of turbulence.The balloon will slowly drift to the edge of space at 120,000 feet (22.7 miles
18、/36.53 kilometers). Here, Fearless Felix will unclip. He will roll back the door.Then, I would assume, he will slowly step out onto something resembling an Olympic diving platform.Below, the Earth becomes the concrete bottom of a swimming pool that he wants to land on, but not too hard. Still, hell
19、be traveling fast, so despite the distance, it will not be like diving into the deep end of a pool. It will be like he is diving into the shallow end.Skydiver preps for the big jumpWhen he jumps, he is expected to reach the speed of sound - 690 mph (1,110 kph) - in less than 40 seconds. Like hitting
20、 the top of the water, he will begin to slow as he approaches the more dense air closer to Earth. But this will not be enough to stop him completely.If he goes too fast or spins out of control, he has a stabilization parachute that can be deployed to slow him down. His team hopes its not needed. Ins
21、tead, he plans to deploy his 270-square-foot (25-square-meter) main chute at an altitude of around 5,000 feet (1,524 meters).In order to deploy this chute successfully, he will have to slow to 172 mph (277 kph). He will have a reserve parachute that will open automatically if he loses consciousness
22、at mach speeds.Even if everything goes as planned, it wont. Baumgartner still will free fall at a speed that would cause you and me to pass out, and no parachute is guaranteed to work higher than 25,000 feet (7,620 meters).It might not be the moon, but Kittinger free fell from 102,800 feet in 1960 - at the dawn of an infamous space race that captured the hearts of many. Baumgartner will attempt to break that record, a feat that boggles the mind. This is one of those monumental moments I will always remember, because there is no way Id miss this.
