© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

6.5.6 Power Analysis in Design

An important part of any nanodevice design exercise is a detailed assessment of power requirements and heat dissipation. Such an assessment might typically include the following elements:

A. Molecular Transport -- Sorting rotors, internal transport mechanisms, sieving or nanocentrifugation, and receptor-based transport have specific power requirements (Chapter 3). Sorting rotors operating at full speed (~10⁵ rev/sec) dissipate ~10¹⁰ watts/m³. However, it should be possible to boost efficiency in some continuous molecular exchange systems by recovering most of the sorting energy by compressing (or concentrating) one species using energy derived largely from the decompression (or deconcentration) of the other via differential gearing.

B. Chemical Transformations -- Drexler¹⁰ estimates that the energy dissipation caused by chemical transformations involving carbon-rich materials is ~1000 zJ/atom or ~10⁸ joules/kg of final product using readily-envisioned irreversible methods in systems where low energy dissipation is not a design objective, but may in theory be as low as ~1 zJ/atom or ~10⁵ joules/kg "if one assumes the development of a set of mechanochemical processes capable of transforming feedstock molecules into complex product structures using only reliable, nearly reversible steps." However, R. Merkle notes that essentially all current proposals are quite dissipative. Most interactions with biological molecules may also have relatively low energy densities because common cut-and-repair operations can involve only a small number of covalent bonds on a single macromolecule. For example, forging just one new 1000-zJ covalent bond on a single 500-residue protein molecule containing ~10,000 atoms has a net molecular transformation cost of only ~0.1 zJ/atom. Transformations involving noncovalent bonds can have even lower per-atom energy costs.

C. Mechanical Operations -- As a crude rule, physically active nanocomponents such as rotating roller bearings generate ~10⁵ watts/m³ of waste heat; inactive parts produce no waste heat. Larger mechanical assemblages such as nanomanipulator arms dissipate ~10⁹ watts/m³ in normal continuous operation,¹⁰ or ~1 megawatt/kg for active diamondoid nanomachinery. Energy required for locomotion, actuation and manipulation is described in Chapter 9; energy requirements for shape-changing or other metamorphic activities must also be assessed (Chapter 5).

D. Communication and Navigation -- There must be a design assessment of the energy required, if any, for internal navigation (Chapter 8) and for all communication tasks (Chapter 7) including intradevice and interdevice signaling, inmessaging and outmessaging, plus allocation of systemic overhead for communications and navigational networks.

E. Computation -- Mechanical nanocomputers using rod logic in the design described by Drexler¹⁰ dissipate ~10¹² watts/m³ at ~GHz clock rates, yielding ~10¹⁰ MIPS/watt or ~10⁴ instructions/sec per pW (~10²⁸ instructions/sec-m³). R. Merkle suggests that the use of nearly reversible computational operations⁷¹³ may reduce energy requirements per instruction by at least two orders of magnitude, at the cost of a slower clock cycle. With standard logic, dissipative irreversible operations can approach a minimum energy dissipation of ln(2) kT ~ 3 zJ per operation at 310 K, and in principle with reversible logic a computer can dissipate arbitrarily less energy per logic operation (Section 10.2.4.1). Nevertheless, total nanorobotic power demand will often be dominated by computational energy requirements (Section 10.2).

F. Component Assembly -- Fabricating nanoscale diamondoid parts using hydrogen-rich organic feedstock molecules and oxygen generates energy, because the process involves a controlled, but highly-exoergic, combustion reaction. In a well-known example given by Drexler,¹⁰ "burning" hydrocarbon to make diamondoid liberates 17 MJ/kg of gross energy, less 1.7 MJ/kg of local entropy decrease in the reactants, giving ~15 MJ/kg of free energy; assuming 3 MJ/kg dissipated in the mills, rotors, and computers needed to drive the manufacturing process, a net output of 12 MJ/kg of surplus energy is produced. However, subsequent higher-order assembly processes using this diamondoid material are endoergic. In the worst case, assembly of larger diamondoid structures from 1 nm cubes may cost ~9 MJ/kg, although with proper technique most of this energy can be recovered as mechanical work, reducing the cost of block assembly of larger structures to as low as ~0.5-1.0 MJ/kg.¹⁰ Leaving aside the initial exoergic energy production, then depending upon computation, materials handling, and other requirements in a particular implementation, assembling prefabricated diamondoid part-like or module-like components into larger structures will probably cost ~13 MJ/kg of energy¹⁰(Chapter 19).

Last updated on 18 February 2003