This article is part of the能源管理Series:探究电源密度
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您将学到什么：
- 将记录直接设置有关功率密度测量。
- Power-density aspects of Li-ion batteries.
- 无线电源传输，风能密度，板载充电器和直流微电网的低调是什么？
Power density is an often confusing and misunderstood concept. Here are 11 myths about power density, followed by the facts that debunk them.
1.A large battery has higher power density than a small capacitor.
False:小电容器可能具有与大电池相同的功率输出，但是电容器的物理尺寸小，使其成为更高的功率密度设备。
For example, a tiny capacitor may have the same power output as a large battery. However, because the capacitor is so much smaller in volume, it has a higher power density.
Why?:电池能够存储更多的能量，但是电容器可以更快地释放能量！由于高功率密度系统迅速释放了能源，因此它们也可以快速充电。
2.Mounting a power transistor to a heatsink on one side of its package for cooling will lead to an optimum improvement in power density.
False:If designers choose a power transistor that can also have a good thermal connection on both sides of its package, then this will further improve the overall system power density and reduce system cost.
Why?:First determine the right size for the heat-spreading copper pad and how many vias will be needed under the thermal pad on the printed circuit board (PCB). If a heatsink can be attached to the other side of the power transistor package, that would optimize cooling and lead to better power density.
3.各向同性天线中的天线功率密度很简单且易于计算。
False:An isotropic antenna/radiator is omnidirectional with uniform power density in all directions. With radar, we now need to determine power density at various distances from the transmitter. We must image the surface area of a sphere (4πR2) at a particular distance R (the range from the antenna which is the radius of the sphere). Since the surface area of a sphere will increase by the square of its radius, the power density (PD), in W/m2，will decrease by the square of the radius.
Therefore, power density from the isotropic antenna is:
PD= Pt(transmitter power)/4πR2
Why?:Power density from an isotropic antenna is complicated enough, but designers also may want to look at radar, which is a subset of isotropic antennas. With radar, we will want to look at power density at some distant point, from an isotropic antenna, multiplied by the antenna gain (Gt)(Fig. 1)。
Radar power density PD= PtGt/4πR2。
4.功率密度测量基于功率设计的面积或体积。查看功率设计的体积，功率密度是处理的输出功率（VO× IO) divided by the area formed by the rectangular area of the PCB (W) and multiplied by the所有组件的平均高度on that board. Power density in terms of area would simply be the power processed (VO× IO）由板的矩形区域（L×W）。
False:Looking at thevolume of a power design, power density is the output power processed (VO× IO）除以由PCB（W）的矩形区域形成的区域，并乘以highest该板上的组件。
5.照明功率密度（LPD）以每平方英尺的瓦特为单位，或者消耗的能量除以空间的大小。瓦数是每单位时间消耗的能量的量度（焦耳/秒）；LPD在他们的计算中使用了这一事实。
False:LPD实际上忽略了这一事实 - 它没有时间组件。LPD是今天的不精确度量，因为它不使用瓦数。
6. Lithium-ion batteries have very high power density.
False:锂离子电池实际上具有相对较低的功率密度小于1 kW/kg，通常为0.15至0.2/kg。超级电容器提供15 kW/kg。甚至还有一种较新的碳离子细胞技术，具有1.5 kW/kg。
7. Wind-power density estimation, for electricity generation based on wind-flow data history in a particular region, is不够准确to determine the size and height of wind turbines to be used in that region.
False:Gathering many years of wind data, at selected region altitudes of some 10 or so meters above surface ground stations, will allow forfairly accurateexamination of the energy potential of those specific locations. Such studies, using the威布尔特征，确定最大空气能量转换为电能所需的最佳能量大小和高度。
8.无线功率传输（WPT）无法将其功率传输增加超过当前标准设计架构。
False:A proven technique of stacking receiver coils actually exists for wireless megahertz power-transfer systems. Multilayer coils are used to improve the equivalent inductance and Q factor of the receiver coil. Next, a corresponding compensation method can be used for the stacked, coil-based power receiver to achieve cross-coupling compensation and full resonance under the varying coupling and load conditions.
9.电动汽车（EVS），带有单独的高功率车载充电器，为驾驶员和乘客提供了更高的范围功能的舒适感。目前，这是加速采用电动汽车的最佳方法。
False:加快采用电动汽车是最好的方法chieve lower weight and increased efficiency, employing a high-power, on-board EV charger into the drive chain. A composite boost dc-dc converter is positioned in the drive train, between the EV battery and the motor drive. This converter consists of multiple power modules: a buck module, a boost module, and a dc transformer (DCX) module(Fig. 2)。
This design concept will enable efficiency and power-density optimization of the overall converter via a control strategy that maximizes the amount of direct power processed by the converter across a wide range of operating conditions. These factors enable the composite boost converter to have a much more improved efficiency and power density than a traditional boost-converter architecture.
10. Energy-storage system designs are best served with lithium-ion batteries.
False:锂离子电池的高能密度高达250 WH/kg，低功率密度小于1 kW/kg。但是，这些电池表现出很高的易燃性。随着时间的流逝，他们的表现迅速消失；与存储的能源量相比，它们的成本很高；如果发生过度充电，可能会非常不安全。
超级电容器在储能方面更好。它们的能源比电池的速度要快得多，并且寿命比锂离子电池更长，但它们也易燃。另一方面，采用纳米碳和离子电解质的碳离子（C-ION）细胞允许在没有火灾风险的较高电压下安全运行。C-ION细胞具有较高的寿命和高功率密度的能力。
11.由于传输线缺乏交流电阻，并且功率转换阶段较少，因此DC微电网提供了高效率。这些微电网具有不同的能源，高度依赖天气条件。由于这些不规则的天气条件，需要能源储存系统（ESS）来帮助不稳定的电源。常规的两端口DC-DC转换器体系结构将一个DC单元连接到一个DC-DC转换器与网格。电源转换器之间需要进行沟通。这是有关系统大小的最佳体系结构。
False:A dc microgrid with three-port dc-dc converters can interconnect two dc units with one converter to the grid with only a small number of components and employ just a single power-conversion stage(Fig. 3)。该系统将更小，更简单，组件较少，从而导致更高的功率密度和较低的成本。可以删除两个端口DC-DC转换器体系结构中的电源转换器之间所需的通信。
结论
电源密度是对特定空间或区域中可以处理多少电源的量度，这是现代电源的必要方面，因为效率至关重要 - 功率密度和效率紧密结合。功率密度实现了促进新应用和新市场的小型化。经济优势也来自更好的功率密度。
Still, power density isn’t well understood by many designers and should not be confused with energy density.
References
Electronic Warfare and Radar Systems Engineering Handbook - Power Density - RF Cafe。
高功率密度动力传动系统集成电动汽车充电器，Usama Anwar，Hyeokjin Kim，Hua Chen，Robert Erickson，Dragan Maksimovic和Khurram K. Afridi。
High Power Density Bidirectional Three-Port DC-DC Converter for Battery Applications in DC Microgrids，Juyoung Sim, Junyoung Lee, Hyunjun Choi and Jee-Hoon Jung
