Archive for June, 2009

For those interested in knowing what I’ve been up to for the last months

Monday, June 22nd, 2009

Considering the complete lack of updates concerning travel and whatnot.

There have actually been a few. And there’s pictures. I’ve just not gotten around to doing anything with it.

I’ve been busy with a Design Synthesis Exercise. The closing project of the Bachelor program here.
While I’m doing that during 08:45 to 17:30 every school day for 2.5 months, I have also been working on school project outside of this timeframe.

Needless to say things have been rather draining.

I joked last post I might add the report, already knowing noone in their right mind would actually be interested in that. Instead, I’ll post the Executive Summary as it is printed in . . . something they printed stuff about all the DSEs of this year.

Before you read this though, there are 2 things you should know.

  1. The real report is 149 pages long. This is a rather compacted view
  2. I didn’t proofread this. Don’t blame me for the poor grammar, dutchisms and spelling errors

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17.1 Introduction

Renewable energy sources are becoming more and more important as fossil fuel reserves are running out or are becoming too expensive. This phenomenon opens new opportunities for renewable energy sources, such as Space Based Solar Power (SBSP). The working principle of SBSP is the generation of power in space using solar cells and subsequently transmitting this power to the Earth by means of wireless power transmission. The main advantages of a space system compared to terrestrial solar conversion systems are a higher surface power density (due to a higher sunlight intensity outside the atmosphere), and a continuous power supply (no day and night cycle).
The mission need statement is formulated as:
“To design a ‘green’ spacecraft (series) to supply renewable solar energy to Earth to be launched before 2025.”
The project objective statement is defined somewhat different:
“Perform a market and technology feasibility study and make a conceptual design for a SBSP harvesting platform by 10 students in 10 weeks.”
The project includes an analysis of the current and future electricity market, a conceptual design of a SBSP system, an analysis on its technical performance, economical aspects and sustainability, and a comparison to Earth based solar farms.

17.2 Requirements

The mission need statement gives rise to a number of requirements for the mission. Additional requirements were set during the design process. The mission requirements of the SBSP mission are:

  • Operational lifetime of at least 10 years
  • Effective power output of 1 GW on Earth at the end of life
  • To be launched before 2025
  • Cost-competitive with other energy sources in terms of $/kWh

Besides these top level requirements, numerous other requirements are defined. These requirements state the criteria on sustainability, safety and (subsystem) design.

17.3 Concept study

For the concept trade-off, three different concepts were considered. In table 17.1 the specifications of the three concepts are given.

Concepts

LEO

GEO

Mirror

Design Options

Transmission

laser

microwave

sunlight

Collection

photovoltaic

photovoltaic

none

Orbit height

1000 [km]

36000 [km]

36000 [km]

Stabilization

active

active

active

Contact time

low

high

high

Nr. of satellites

multiple (± 50)

1

1

Nr. of ground stations

multiple (± 50)

1

1

Ground station type

existing farms

custom rectenna

existing farms

Weather attenuation

high

low

high

Table 17.1: SBSP concept characteristics

LEO concept

The LEO concept consists of a constellation of multiple satellites in LEO. For the collection of solar energy photovoltaic cells are used. A laser system is used for the transmission. The stabilization of the system has to be performed actively, since the lasers rotate. An advantage of this concept is that the lasers can be transmitted to already existing photovoltaic farms, reducing cost. Secondly, lasers are smaller and lighter than microwave antennas. Moreover, the orbit can be chosen to deliver energy to Earth to serve during energy peak moments in the morning and evening. A major drawback of the LEO concept is the large amount of ground stations and satellites needed to provide energy continuously, as the LEO satellites have a short contact time with a ground station.

GEO concept

The GEO concept consists of a single GEO satellite, which is coupled to its own ground station. The system can easily be scaled with other GEO satellites and ground stations. For collection of solar energy photovoltaic (PV) cells are used in combination with solar concentrators, increasing the solar intensity. A microwave system is used for the transmission. The satellite stabilization has to be performed actively, since the PV cells have to rotate to the Sun. Advantages of the GEO concept are that the system is stationary above ground and microwaves are barely influenced by clouds. A disadvantage is that a large amount of mass and volume has to be launched to GEO height, which requires many launch vehicles, increasing the overall cost.

Mirror concept

The space mirror concept consists of a single satellite in GEO and a corresponding ground station on Earth. The satellite is a mirror reflecting the solar radiation towards the ground station without converting it to other forms of energy. The concept excels in simplicity in the absence of a collection system, transmitting devices and cabling. As a result the complexity of the system and values for the mass and volume can be made significantly lower than for other design concepts. Just as the LEO concept, the mirror concept can make use of already existing photovoltaic farms, even more efficiently. A major drawback of the mirror concept is that it requires ‘soller slits’ to create parallel solar beams for decreasing the spot on Earth to a reasonable size. Such soller slits exclude a large part of the incoming sunlight, decreasing the overall efficiency significantly.

Trade-off

In the trade-off the disadvantages of all three concepts were expected to be too problematic and it was decided to come up with a concept combining the advantages of the three designs. This final concept is named after the courier of the Sun in Mithraic ceremonies: ‘Heliodromus’.

17.4 Detailed Design

Heliodromus consists of ten satellites in a Sun-synchronous LEO and two reflecting satellites in GEO. A single GEO satellite has five mirrors, one for each LEO satellite. The LEO satellites collect the Sun’s energy and transmit it by means of lasers to the mirrors in GEO, which redirect the light to Earth in return (figure 17.1). Heliodromus has the advantage of having only little mass in GEO, as most of it is in LEO. The contact time with the ground stations is permanent, i.e. the energy supply is continuous. Moreover, the system is easy scalable, requiring only a small mirror satellite to be launched to GEO for each new LEO satellite.

SchematicOverview

Figure 17.1: Schematic overview of Heliodromus

For an end-of-life power output of 1 GW on Earth, a total surface area of 3.44 km2 of thin film photovoltaic cells is needed per LEO satellite. The photovoltaic conversion is done with a 15% end-of-life efficiency. Thin film panels are used for collection in space, as thin film is cheaper to produce and has a larger power and package density than other PV cells. Consequently, less launches will be needed. Another advantage of thin film is that it is easy to assemble and maintain, which will reduce the overall cost.

The generation of the laser beam for wireless power transmission is done with an efficiency of 60% and each individual laser has a power output of 1 MW at a wavelength of 1064 nm. In total, the system of ten LEO satellites uses 4000 lasers. The choice for laser on Heliodromus instead of microwaves transmission is made, because of the huge antennas and rectennas required to generate power in the order of Giga-Watts. Moreover, already existing studies on the subject by NASA and ESA only covered microwave, hence a concept using laser transmission was open for out of the box thinking.

A mirror system on the LEO satellites, using mirrors with a diameter of 9 meter and 12 meter, bundles all separate lasers rays to a single beam and directs it to the GEO satellite (figure 17.2). The LEO satellites have a permanent contact time with one of the two GEO satellites, switching between them when needed. The GEO satellite, having five mirrors of 24 m diameter each, reflects the energy to the associated ground station, providing energy to Earth continuously.

LaserTransmission

Figure 17.2: Laser transmission from LEO to GEO

The laser beam travels through the atmosphere with an efficiency of 85% and reaches the Earth’s surface at two ground stations, located in Arizona, USA and Egypt, North Africa. These areas are selected considering cloud coverage and aerosol density, seismicity and political stability.

At the ground station the laser beam is converted by means of monochromatic photovoltaic cells, optimized for the wavelength of the laser. This conversion can be performed with an efficiency of 40%, resulting in a power output of 500 MW per ground station. The ‘Stirling engine’ is the best option for solar dynamics, with a conversion efficiency of 31%. The silicon cell proved to be the best photovoltaic option, with a conversion efficiency of about 40%. Together with the fact that photovoltaic cells are used more often than heat engines and consequently these cells can be implemented faster, makes photovoltaic conversion favorable.

17.5 Performance

The performance of Heliodromus can be evaluated by looking at different aspects, namely the overall efficiency, technical readiness levels, the energy payback time and the total costs. A performance comparison between Heliodromus and an Earth based solar farm is made to see whether Heliodromus can be competitive with more conventional renewable energy sources.

Overall efficiency

The overall efficiency of the Heliodromus concept is estimated to be approximately 2%, based on a power input and output of respectively 47 GW and 1 GW. The major losses occur during the initial energy conversion by the photovoltaic thin films in space, which have an efficiency of only 15%. The conversion of electricity to laser is done with an efficiency of 60%. The conversion on ground also significantly contributes to the losses, with an efficiency of 40%.

Total costs

To be market viable the electricity price of Heliodromus must be around $7-8 cents/kWh, as concluded in a market analysis. The total cost of Heliodromus is $98 billion, of which nearly $80 billion is the costs of the laser devices and the launches. The resulting electricity price of Heliodromus is approximately $1/kWh. This value is based on the most pessimistic mass estimates, i.e. the highest launch costs with a total of 1200 launches.

Energy payback time

The energy payback time for the Heliodromus concept is estimated using the following division: the collection system, the transmission system, the ground station and the launch. This led to a total energy payback time of approximately 6 years.

Technical readiness levels

The main bottlenecks in the used technologies are the laser and the thermal subsystems. The required 1 MW continuous wave lasers are not yet available and especially not with a lifetime of 10 years or higher. Moreover, the 60% efficiency of the laser results in 40% energy loss, i.e. waste heat. This waste heat has to be rejected by means of thermal radiators. The amount of radiating surface needed is high, contributing for 50% to the mass and volume of a LEO satellite. Therefore, improvements on either the efficiency of the laser or the effectiveness of the thermal radiators must be made. Furthermore, to increase the overall efficiency of Heliodromus, the efficiency of photovoltaic cells need to improve.

Another major concern for Heliodromus is the assembly of the satellites in space. The dimensions of both the LEO satellites and the reflectors in GEO are of such an order that they do not fit into a single launch vehicle. For this reason assembly in space is required to create the complete structure. Since the assembly involves thousands of parts, the assembly of these parts and the realization of the large amount of launches in a short time frame will become a big challenge.

Comparison with Earth based solar farm

Compared to Heliodromus, an Earth based solar farm with an equal solar panel area generates approximately 125% more energy, assuming an extended lifetime of 15 years for the space based system and 20 years for the Earth based system (degradation of photovoltaic cells in space is higher). This difference occurs when the thin film cells used for energy collection in space are put in Arizona, USA on ground. The difference between the Earth and space based solar plant is that the space based system has 1.5 km2 of solar panels on Earth, while the Earth based system has 34.4 km2 of solar panels.

When comparing Heliodromus to already existing solar farms, both photovoltaic and solar dynamic (SD), Heliodromus is 5-10 times as expensive as already existing Earth based systems (table 17.2). For an Earth based system the energy payback time ranges between 1 to 2.7 years, compared to the 6 years for Heliodromus.

Average Output

Costs

Lifetime

Output

[GWh/year]

[million $]

[years]

[$/kWh]

Heliodromus (PV)

8800

89000

10

1.01

Moura Spain (PV)

93

325

20

0.17

Waldpolenz Germany (PV)

40

170

20

0.21

Andasol Spain (SD)

180

390

20

0.11

Nevada Solar One (SD)

134

266

20

0.10

Table 17.2: Comparison with existing Earth based solar farms

17.6 Conclusions and recommendations

Heliodromus is a result of a concept exploration. By means of a subsystem analysis three concepts were proposed. Trade-off between these concepts resulted in the selection of a fourth concept, as a combination of the other three.

The Heliodromus concept is a constellation consisting of ten satellites orbiting in LEO and two modules orbiting in GEO having five mirrors each. The number of satellites in LEO is scalable depending on the energy demand on Earth. For each added LEO satellite a mirror must be launched to GEO. The LEO satellites are in a 1400 km orbit and the height of a GEO is 35786 km. The locations of the ground station were determined by evaluating cloud and aerosol densities, seismicity and political stability of the region. The trade off between these criteria resulted in the selection of two regions: Arizona, USA and Egypt, North Africa.

An estimate of Heliodromus’ total efficiency resulted in a value of 2%, which is not sufficient to compete with Earth based solar systems. Heliodromus’ energy payback time is 6 years, which is within the minimum lifetime of 10 years, but still longer than the energy payback time of the Earth based system. The total costs of Heliodromus are around $98 billion and at this stage it can not be price competitive with Earth based fossil nor renewable energy sources. When the complete amount of thin film arrays used by Heliodromus is installed on Earth this system delivers 125 % more energy. Existing solar farms are already 5 to 10 times cheaper, at around $10-20 cents/kWh compared to the $1/kWh for Heliodromus.

Component

Efficiency [%]

Future Efficiency

Thin Film in space

15

30

Lasers

60

70

PV cells on ground

40

60

Other

60

60

Overall Efficiency

2 %

8 %

Table 17.3: Subsystem efficiency and possible improvements

The major technological bottlenecks are the laser efficiency and performance. Coupled to this problem is the size and mass of the thermal radiators required for the LEO satellites. Moreover, developments in PV cell efficiency would significantly reduce the required mass and volumes for launch to space. In addition, assembly in orbit has never been done on the scale needed for Heliodromus and it opens a totally new field of research and development. Current efficiencies and possible future efficiencies are given in table 17.3. The total efficiency can be improved up to a factor 4 in near future, getting Heliodromus closer to market viability.

Final Report

Thursday, June 18th, 2009

The project is finished.

Well, the symposium is tomorrow, but it’s pretty much done.

Front page of Final report

Front page of Final report

I got the pdf of the entire report. Maybe I’ll upload it, though I seriously doubt anyone would actually be interested in a 150 page technical manual

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Quote of the day:

But luckily all these problems have a solution
*click*
solution

Damn, that was probably not what I was supposed to say with that slide

– Project member X while practicing final presentation in front of the group

Design Synthesis Exercise

Thursday, June 11th, 2009

artist_impression_1

I do useful stuff