Tuesday, 11 June 2013

Solar Power Satellite II

NASA berpendapat cara pembuatan Satelit Tenaga Surya itu cukup realistis sehingga mereka mendanai kelompok Artemis Innovation Management Solutions untuk mengembangkan Solar Power Satellite via Arbitrarily Large Phased Array (SPS-ALPHA). Satelit ini akan berbentuk tulip dan dilengkapi film tipis cermin untuk memantulkan sinar matahari ke dalam sel photovoltaic. 

Energi matahari yang terkumpul akan diubah menjadi gelombang mikro, kemudian dikirim kembali ke stasiun penerima di Bumi dengan frekuensi dan intensitas rendah. Pembangkit tenaga listrik di Bumi akan mengubah energi microwave menjadi listrik dan menambahkannya ke jaringan listrik. 

NASA mengatakan bahwa setiap susunan kaca bisa saja menghasilkan puluhan hingga ribuan megawatt energi.

Spacecraft Design

An SPS essentially consists of three parts:

  1. a means of collecting solar power in space, for example via solar cells or a heat engine
  2. a means of transmitting power to earth, for example via microwave or laser
  3. a means of receiving power on earth, for example via a microwave antennas (rectenna)

The space-based portion will be in a freefallvacuum environment and will not need to support itself against gravity other than relatively weak tidal stresses. It needs no protection from terrestrial wind or weather, but will have to cope with space-based hazards such as micrometeorites and solar storms.

Solar energy conversion (solar photons to DC current)

Two basic methods of converting photons to electricity have been studied, solar dynamic (SD) and photovoltaic (PV).

SD uses a heat engine to drive a piston or a turbine which connects to a generator or dynamo. Two heat cycles for solar dynamic are thought to be reasonable for this: the Brayton cycle or the Stirling cycle. Terrestrial solar dynamic systems typically use a large reflector to focus sunlight to a high concentration to achieve a high temperature so the heat engine can operate at high thermodynamic efficiencies; an SPS implementation will be similar.

A major advantage of space solar is the ease with which huge mirrors can be supported and pointed in the freefall and vacuum conditions of space. They can be constructed from very thin aluminum or other metal sheets with very light frames, or from materials available in space (eg, on the Moon's surface).

PV uses semiconductor cells (e.g., silicon or gallium arsenide) to directly convert sunlight photons into voltage via a quantum mechanical mechanism which evades the thermodynamic limitations on heat engines. Photovoltaic cells are not perfect in practice as material purity and processing issues during production affect performance; each has been progressively reduced for some decades.

These are commonly known as “solar cells”, and will likely be rather different from the glass pane protected solar cell panels familiar to many which are in current terrestrial use. They will, for reasons of weight, probably be built in a membrane form not suitable to terrestrial use where the considerable gravity loading imposes structural requirements on terrestrial implementations.

It is also possible to use Concentrating Photovoltaic (CPV) systems, which like SD are a form of existing terrestrial Concentrating Solar Energy approaches which convert concentrated light into electricity by PV, again avoiding the thermodynamic constraints which apply to heat engines. On Earth, these approaches use solar tracking systems, mirrors, lenses, etc to achieve high radiation concentration ratios and are able to reach efficiencies above 40% Concentrating Photovoltaic Technology.

Because their PV area is rather smaller than in conventional PV, the majority of the deployed collecting area in CPV systems is mirrors, as with most SD systems. They share the advantages of building and pointing large (simple) mirror arrays in space as opposed to more complex PV panels.

Phase I Final Report (PDF)
SPS-ALPHA: The First Practical Solar Power Satellite via Arbitrarily Large PHased Array 
By: Dr. John Mankins, Artemis Innovation Management Solutions
Comparison of PV, CPV, and SD
The main problems with non-concentrating PV are that PV cells continue to be more expensive relative to the other approaches, and require a relatively large area to be acceptable for a significantly sized power station. In addition, semiconductor PV panels will require a relatively large amount of energy to manufacture; amorphous-silicon designs require much less energy to produce but have been substantially less efficient. CPV designs with a small area of 40%+ efficient cells and large reflector area are expected to be less expensive to produce.

As well, the materials used in some PV cells (eg, gallium and arsenic) seem to be less common in lunar materials than is silicon; this may be significant if lunar manufacturing is involved.

SD is a more mature technology, having been in widespread use on Earth in many contexts for centuries. Both CPV and SD systems have more severe pointing requirements than PV, because most proposed designs require accurate and stable optical focus.

If a PV array orientation drifts a few degrees, the power being produced will drop a few percent. If an SD or CPV array orientation drifts a few degrees, the power produced will drop very quickly, perhaps to near zero.

Aiming reflector arrays requires much less energy in space than on Earth, being without terrestrial wind, weather, and gravitation loads, but it has its own problems of gyroscopic action, vibration, limits on usable reaction mass (though electrically powered gyros would avoid that problem), solar wind, and meteorite strikes on control mechanisms.

Currently, PV cells weigh between 0.5kg/kW and 10kg/kW depending on design. SD designs also vary but most seem to be heavier per kW produced than PV cells and thus have higher launch costs, all other things being equal. CPV should be lighter; since it replaces the thermal power plant (except for a radiator for waste heat) with a much lighter PV array.

Kunjungi Juga:

Pusat Teknologi Satelit - Lembaga Penerbangan dan Antariksa Nasional

Semoga Bermanfaat.

To Be Continued 

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