课讲那么复杂

贵校出版社出的教材又那么落后于时代

搞得我们人心惶惶

觉得这课复习起来跟个文科一样

最后您卷子一出

又跟信号一样

白白复习了那么多文字

PPT跟百科全书一样

没有啥逻辑

复习范围没有个着落

各年考题五花八门

明天去考场以后究竟会如何呢？

保佑啊保佑

刚才突然想拍下计原实验箱留个纪念，家里的相机没有带过来，于是借玮玮的IXUS90IS来用。

开始拍摄角度还比较正常，拍着拍着就变成和那堆元件玩了。。。一口气拍了31张，感受了下小DC的IS和微距。照片正在往Live Photo Gallery里传。

IS的实用性是毋庸置疑的，拿着350D在室内拍摄，怎么也得1/60s的快门才敢下手，毕竟ISO高过800以后惨不忍睹。这回在中厅，光线不怎么好，只敢把ISO调到400（后来发现还是有点高了。。。），然后用慢快门配合IS来拍。1/6s的情况下拍出来还是能看的，0.4s的话就控制不住手抖了。

这个小相机微距性能还是可以的，如果光线好的话，2cm左右还是可以准确对焦的，只是焦外。。。很惨。

计原实验不愧是这学期的一个杀手，除了一只手能数过来的少数组之外，几乎每个人都不得不取实验室通宵调程序。

今年实验难度挺大，原因各种各样。

比如说这次正好赶上这个实验改革，跟以前不一样了。n年前做8位机，我们做16位。n年前做多周期的，我们做流水。而且流水做出来才80分，还得做旁路做分支预测做中断处理甚至连Cache都在实验扩展的列表里。又因为是改革中的实验，文档乱七八糟，实验箱电路图没给我们，实验指示书给的是改革前的版本，很多地方与今年的机器不符。n个通宵之后，原订的deadline之后，助教突然发文说实验箱上标着某个编号的pin其实不是那个号，而真实的编号对应的pin是FPGA保留的，不对用户开放。又比如再过了几天助教又说实验指导上关于控制信号的给定说明有错误。还比如很多组都做完了助教又冒出一句实验箱某个输出的低电平有一点几伏，估计硬件故障……

这只是硬件上的，软件方面，编译器把0x0000当做nop指令，而实际我们的指令集里还真有0x0000这条指令，实验都开始两周多了，师兄们在闲谈中突然得知原来0x0000不是nop啊，发文：所有人改！过一阵子，有人发现改写term加延时就能跑出正确结果，否则不行。于是井上发布更新版term，可是此前已经有一些组用原版term验收过了丫，怎么能怪term呢？

天天实验，停课做实验，早也做晚也做，就是没个结果，当文档慢慢补全的时候，实验进度也慢慢跟上来。最后看到自己的cpu成功输出一行welcome字样的时候，3个人当场大叫。谁知道紧跟着跑fibonacci就立马浇盆凉水下来。这是最后一道坎了，这一道坎让我们明白，要勇敢地质疑硬件！出了问题，首先相信自己的程序，换rom，换ram，换8251，换下载线，实在不行整个机器抛弃掉，请来传说中的“
神机”，这不，同样的程序就过了！

一个计原实验拖了这么久，其实现在3个人都还没着手写实验报告，但是感觉这个实验已经过去了。

在对实验越来越熟悉的过程中，也越来越佩服那第一个做出来的组。人家就撇开那残缺加错误的文档，自己拿个万用表一个管脚一个管脚地量，量出正确的做法来了，这其中的汗水想必不比我们跟着错误的文档走弯路的少。

这个实验，一方面让我们看到神和人的差别之大，就是有人一直不来实验室，我们做到一半，他们抱着机器来了，一遍过！另一方面也让我们感受到牛和人的差别之小，我原想着傍上两个牛就高枕无忧了，谁想还是做得如此辛苦，也有大牛抱团但是做得比我们组还要慢还要坎坷的。运气使然啊，我们自从出了welcome之后就没有单步调试过，换个神机就好了，有些牛人单步调得辛辛苦苦，几百上千条指令下来都跟simulator一样，但是就是不见输出，这倒是我们从未遇到的。

实验就这么过去，期末马上来临，明天去参加那冗长的清华大学第38次学生代表大会，花整整一天时间耗在大礼堂，就为了选一个校会主席，而实际上我们的选票并不能决定谁是主席，这种真的伪民主何必要拿上台面来呢？周三presentation，讲从没听过的“气雾显示技术”，周四考公共财政学，周六考计量经济学，还有一对作业报告和一篇期末论文，元旦过后就是最后的决战了，人智信号计原编译自动机，实在有点没底，这闹腾了一学期，到底这些课结局如何，现在一点都没法预测。。。

周二考100m蛙泳，距离是肯定们问题的，时间。。。不知道诶

哟，照片传好了，就此收笔。

考试两周，一共6门，不算多也不算少。少的前天已经考完了；多的，20号晚上最后一门。唉，我们寝那可怜的娃。。。

Lucifer刚才来我们寝说他魔电复习了两周，把书上题目都做了一遍，但还是被whb算计了。

我们很镇静地对他说只复习了两天就去考了。多复习两周拿额外的10分，对他很重要，对我们，目前没有饱览群书重要。

魔电么，从考完后那么多人壮烈牺牲的局面来看，过程其实并不重要。老师想给你什么，你就只能得到什么。

图论和代数结构也证明这一点，平时学得好又怎样，平时题目都不会做又怎样。

反正考到你会的你就100，考到不会的，十几分十几分地扣嘛。

多少人倒在最近邻算法前，书上没有，课上讲了。

我是记得你课上讲了，但是我只记解决问题的算法，至于这算法叫什么名字，没记。

中微的考试么。。。就复习了一个下午，晚上去考。

这助教也真是的，说好早上把最后一次作业答案传到讨论区的。我想嘛，他传好的时候也就是我做好作业的时候。

谁知道他们。。。到下午才传，晚上7点考试唉。。。给这么点时间复习。。。

还好，毕竟传上来了，对了下答案果然有错的。

错了就改咯。

改了晚上就会做了，啊哈哈哈哈哈哈哈哈~

其实我也很纳闷，为什么出的题我好像都会做呢？

为什么呢？

难道是因为我没去听过课？只去考了两次期中考试？

不应该啊。。。

接下来的微积分，听名字还以为是理科考试。事实证明是考背诵的。。。

只要把她泄的题的证明都背下来，就没问题。可是，我背得下来么？

这么久不碰，连微分方程都不会解了。

还有物理和复变。

量子物理到现在还是有点迷迷糊糊，复变就不说了。

显然是个很漂亮的体系。但是数学中凡是美到一定境界的东西，那就不是让我这种人看的嘛。

反正

加油！

21号我就出发啦~

21号我就到啦~

15号就要去机考啦。。。

22号就得回来了。。。

24号就要开学了。。。

What drives us to the future?

A Brief Look into Solar Energy & Fusion

2006011291| Qing Pei

Department of Computer Science & Technology, Tsinghua University, Beijing, PRC, 10084

Abstract: This paper mainly discusses about solar energy and fusion and their perspective as future sources of energy. With a look into both ways of energy conservation, a comparison on several aspects is given. The author’s attitude toward the future use of the two sources of energy is finally stated based on the foregoing discussion.

Keywords: energy, solar energy, fusion

Table of Contents

Prolegomenon. 3

What do We Need?. 3

Solar Energy: right around us. 5

An Inexhaustible Energy Source. 5

How do we collect solar energy?. 5

Fusion: a tiny sun. 7

The Real Sun versus The Artificial One. 11

Conclusion. 12

Bibliography. 12

Acknowledgement 12

Prolegomenon

The world is developing faster than ever. We need more and more energy to drives us to the future. Some scientists have already pointed out the problem facing us, energy shortage, as early as several decades ago. The exhaustion of the fossil fuels will soon come. We need a new energy source. It is expected to be green, stable and conservable. Some advances in this field have been there so far. Two of the optional energy sources, solar energy and fusion, I think, is important. Here I want to scan these two sources and compare them. The conclusions, I hope, either proved or disproved in the future, can help me understand the situation better.

What do We Need?

What do need to solve the problem? Simply, we need a new source of energy. The quality of the energy is still unclear in this answer. Some further look into the problem will help us see what we really desire.

How much energy do we need? This is a difficult question to answer. Figures in the past, however, can be based on to predict the future. Here is one.

Country

1975

1980

1990

2000

USA

15.0

20.0

40.0

80.0

USSR

7.5

15.0

35.0

70.0

Germany

3.0

5.0

10.0

20.0

France

2.5

4.0

8.0

15.0

Canada

3.0

5.0

10.0

20.0

Japan

2.5

3.5

7.0

15.0

UK

2.0

3.0

5.0

10.0

India

0.5

1.0

4.0

16.0

China

0.1

2.0

6.0

20.0

Other Countries

13.9

21.5

35.0

60.0

Total

50.0

80.0

160.0

326.0

Table 1 Global estimated energy consumption in trillion KWH[1]

Here is another.

Critical Global data

1995

2040

Population

5×10⁹

10×10⁹

Energy consumption

350EJ

900EJ

Average individual energy consumption

2200W

3000W

Table 2 Situation analysis 1995 and prognosis of 2040[2]

From both tables, we can see the energy consumption is unbelievably huge. Another alert is the consumption has more an exponential increase than a linear one. This is making the problem more urgent.

What kind of energy? We are using fossil fuels almost everywhere in the world. We depend on it so much that the day it is used out will be like a doomsday to us. It won’t be long before that day. In fact, some people have even told us the estimated date. They give us, though their estimations vary, the same signal, we must have an INEXAUSTIBLE source of energy to rely on. At least it should leave the work for finding its substitute for another hundreds or thousands of years. The rapid development of human civilization will be sustainable only in this way.

Another fatal defect of the fossil fuels is the exhaust gas. The carbon inside the fossil fuels turns out to be CO₂ in the air. This can be tragic. We are giving out more CO₂ than the biosphere can deal with. The future energy should, therefore, be GREEN.

Besides, the collection, conversion and transmission of the energy must be feasible and economical since we are to use the energy in a large scale. Efforts should be made to ensure both the rich and the poor can afford the new source of energy.

Conclusion: We need an inexhaustible green source of energy to meet the booming demand for energy, the application of which should be both feasible and economical.

Solar Energy: right around us

An Inexhaustible Energy Source

The earth receives solar energy in the form of electromagnetic radiation. The solar energy density is about 1.353kW/m² on the outer surface of the earth’s atmosphere[3] and up to 1.0kW/m² on the lithosphere[4]. That is to say, the total sum of solar energy we get, even if we can only utilize as little as 1% of it, seems “too” much for us.

How do we collect solar energy?

If we just need heat, put something cool out in the sunshine and it will collect the energy. However, hot things cannot be heated this way. Neither can the process go far enough till it reaches the state we need. The efficiency of such a thermodynamic process is pretty low. The Carnot efficiency is

In the collection above, T_high (temperature of the source) is not much higher than T_low (temperature of the sink).

This low efficiency results from the low energy density. We must concentrate the energy to one point, or to make T_high there much higher, to get higher power, or higher efficiency, needed in application. A simple method is to use lens. A magnifier is enough to heat a paper to its burning point. This might make some readers recall the childhood times if they had done such a little “physics experiment”. To make a bigger concentrator, “a parabolic reflector gives high concentration.”[5]

Electricity is easy to transmit and distribute. Solar energy, as a result, is expected to be converted to electricity in many cases. This conversion can be done in different ways.

Suppose the efficiency of each step is η₁, η₂ …, the overall efficiency is

The final energy we have in the form of electricity is

Let the efficiency of the direct conversion be η₀. The final energy we have is

We have to work to convert the energy from one form to another. Each process is not 100 percent efficient, either. Normally, the more process steps, the more energy is lost.

We can draw a conclusion:

Therefore, the direct conversion is the choice. (The technical problem is yet to be solved.)

Photovoltaic cells, or solar cells, are usually used to convert solar energy to electricity. P-N junction solar cells catch the photons from sunlight. The energy absorbed makes hole-electron pairs near the P-N junction. The internal electric field forces the holes and electrons to displace in a certain direction. Voltage is generated. With the load connected, electron current flows and the load works.

Loferski’s formulation describes the current-voltage relation of the P-N junction:

where I_sc is the current density; N_ph the number of incident photons of frequency ν; E_G the energy gap.

The open circuit voltage is

where I₀ is the equal and opposite current in either direction at equilibrium and

With the increase of E_G, V_oc increases while I_sc decreases. A maximum value of is decided by E_G. To get a cell with higher voltage, we link the cells.

The maximum efficiency of a solar cell is 0.25 theoretically. Organic semiconductors do not need to be made in single crystal form. The disadvantage is their relative lack of efficiency (about 1%).[6] The lifetime of a cadmium sulphide cell ranges from 2 to 60 years.[7]

Despite the actual lower efficiency we can reach now, solar cells are more and more used in calculators and in such areas as Tibet and Inner Mongolia where solar energy is rich but the power supply insufficient.

Fusion: a tiny sun

Long before we tried to use the energy of fusion reaction, the stars had been using it. Fusion is now chosen as a future source of energy. It should be since it was the natural one.

The universe consists of more than 99% plasma[8] though it is only 3 Kelvin on average. Have a look at the starry sky, all the twinkling stars are giving out light because of fusion reaction.

On Nov 1, 1952[9], the first hydrogen bomb exploded. Human beings began the use the energy of fusion reaction.

The fusion of the hydrogen isotopes deuterium (D) and tritium (T) according to the reaction

produces 17.6MeV of energy.[10]

Other fusion reactions include

The fusion reaction rate per unit volume can be written

where n₁ and n₂ are the densities of species 1 and 2, respectively, and

is the fusion reactivity. Here v is the velocity, f is the velocity distribution function, and σ_f is the fusion cross section. It is usually adequate to use a Maxwellian distribution,

to evaluate ⧼σv⧽, in which case the value of the integral depends only upon the temperature T of the plasma.[11]

The energy released in most fusion reaction, e.g. an H-bomb explosion, is too much to control. For peaceful use, the reaction must be under control. We have to make the energy flow stable. One method is the magnetic confinement.

In magnetic confinement, the ions move in a toroidal path in the outer magnetic field. The Lorentz force propels them to orbit in a round path. The electric field force makes it move in the normal direction of the surface which includes the round path.

The movement can be described by the following equations:

where F_E is the electric field force, F_L the Lorentz force, q the charge of the ion, m the mass of the ion, B the magnetic field intensity. Its path is shown in Figure 2[12].

Figure 3 Schematic design of stellerator plasma and magnetic coils[13]

Figure 4 Plasma in the ASDEX Upgrade tokamak[14]

Another method is the inertial confinement. The principle is simple: let the reactants react before the explosion. The realization is, however, difficult.

Unlike fission, fusion reaction has no critical mass. This makes small scale use of the energy of fusion reaction possible. Another problem comes: the Lawson Criterion for the commencement of fusion:

where n is the density of the fuel (n particles per unit volume), and τ is the minimum time during which they are confined together.[15]

Confinement Method

Magnetic Confinement

Inertial Confinement

n(cm^-3)

10¹⁴~10¹⁶

10²⁴~10²⁶

Τ(sec)

0.01~1

10^-10~10^-12

Figure 5 Comparison between the two confinement method concerning n & T[16]

Either the relatively long T in magnetic confinement or the vast n in inertial confinement is reached the commencement condition at the moment.

There is still another obstacle in making a fusion power plant. The plasma must be heated to a certain temperature, (or compressed to a certain density), to start the fusion reaction. Scientists have made laser guns and ion guns to provide the energy. Among them, NOVA in the US and AURORA in Russia can produce over 100kJ less than 1ns, fascinating but not enough for application. The dispersion and relatively low efficiency both make the task hard. The cost is too much as well.

The Real Sun versus The Artificial One

Source

Solar energy

Fusion

Availability

☆☆☆☆☆

☆☆☆☆☆

Power plant lifetime

☆☆☆☆

☆☆☆☆☆

On-the-go power supply

☆☆☆☆

(Tiny solar cells)

☆☆☆

(Stored in other media)

Safety Index

☆☆☆☆☆

☆☆☆☆

Pollutants after use

Zero

Zero

Maximum Power

☆☆☆

(Need concentration)

☆☆☆☆☆

(As the H-bomb shown…)

Cost

☆☆☆

(Lots of panels needed)

☆☆☆☆

(Complex design & a control system)

Feasibility

☆☆☆☆

(Already in use)

☆☆☆

(In labs but progresses made)

Figure 6 Comparison of solar energy and fusion

Both sources of energy seem, at least now, inexhaustible. The power plants both can last decades. With solar cells, we can gain solar energy on the go; confined fusion reaction needs a complex system, which seems impossible under the current technical condition. Solar energy is quite safe. The output of fusion reaction, in contrast, needs to be controlled to a relatively lower state than it is in the explosion of a hydrogen bomb. Otherwise it will probably cause danger. Neither the use of solar energy nor that of fusion produces pollutants as CO₂, NO_x or other exhaust gas. The resultant is just electron flow or helium. Fusion, without the restriction of critical mass, is better than fission in a power plant. It produces more energy per unit mass of reactant. The cost of building a power plant is a problem. A solar power plant takes up a large space since lots of solar panels are in it.

Both choices, however, need further improvement before practical use.

Conclusion

Solar energy and fusion both have a bright perspective. The two inexhaustible sources of energy will, once working, relieve the energy crisis. Research in this field is still advancing.

I tend to choose fusion as the main source of energy, as the alternative for heat-engine & hydroelectric plants, for it has an ample power output. Solar energy can work as a supplement in rural areas and for mobility use.

Bibliography

Agarwal, M. (1983). Solar Energy. New Dehli: S. Chand & Company Ltd.

Arons, J. d. (2007, March 15). The Metabolic Society. Beijing, China.

Bockris, J. O. (1980). Energy Options. New York: Halsted Press.

Ganchang, W. (2000). The Small Artifitial Sun, Inertial Confinement Fusion. Beijing, Tianhe: Tsinghua University Press, Jinan University Press.

McVeigh, J. (1984). Sun Power: An Introduction to the Applicaions of Solar Energy. (X. Wang, & X. Li, Trans.) National Defense Industry Press.

Weston M. Stacey, J. Fusion: an introduction to the physics and technology of magnetic confinement fusion.

Wilhelmsson, H. (2000). Fusion: A Voyage Through the Plasma Universe. London: Institute of Physics Publishing.

Acknowledgement

Special thanks to Qian Wenwen for suggestions in the composition of some sections and proof of the whole paper.

[1] (Agarwal, 1983, p. 3)

[2] (Arons, 2007, p. 13)

[3] (McVeigh, 1984, p. 15)

[4] (McVeigh, 1984, p. 19)

[5] (Agarwal, 1983, p. 33)

[6] (Bockris, 1980, p. 163)

[7] Table 8.IX (Bockris, 1980, p. 163)

[8] (Wilhelmsson, 2000, p. XV)

[9] (Ganchang, 2000, p. 60)

[10] (Weston M. Stacey)

[11] (Weston M. Stacey, p. 2)

[12] (Weston M. Stacey, p. 5)

[13] Plate 13 (Wilhelmsson, 2000)

[14] Plate 14 (Wilhelmsson, 2000)

[15] (Bockris, 1980, p. 100)

[16] Edited with reference to (Ganchang, 2000, p. 66)