Stephen H. Salter

Sea-going hardware for the cloud albedo method of reversing global warming

Following the review by Latham et al. (Latham et al. 2008 Phil. Trans. R. Soc. A 366) of a strategy to reduce insolation by exploiting the Twomey effect, the present paper describes in outline the rationale and underlying engineering hardware that may bring the strategy from concept to operation. Wind-driven spray vessels will sail back and forth perpendicular to the local prevailing wind and release micron-sized drops of seawater into the turbulent boundary layer beneath marine stratocumulus clouds. The combination of wind and vessel movements will treat a large area of sky. When residues left after drop evaporation reach cloud level they will provide many new cloud condensation nuclei giving more but smaller drops and so will increase the cloud albedo to reflect solar energy back out to space. If the possible power increase of 3.7 W m−2 from double pre-industrial CO2 is divided by the 24-hour solar input of 340 W m−2, a global albedo increase of only 1.1 per cent will produce a sufficient offset. The method is not intended to make new clouds. It will just make existing clouds whiter. This paper describes the design of 300 tonne ships powered by Flettner rotors rather than conventional sails. The vessels will drag turbines resembling oversized propellers through the water to provide the means for generating electrical energy. Some will be used for rotor spin, but most will be used to create spray by pumping 30 kg s−1 of carefully filtered water through banks of filters and then to micro-nozzles with piezoelectric excitation to vary drop diameter. The rotors offer a convenient housing for spray nozzles with fan assistance to help initial dispersion. The ratio of solar energy reflected by a drop at the top of a cloud to the energy needed to make the surface area of the nucleus on which it has grown is many orders of magnitude and so the spray quantities needed to achieve sufficient global cooling are technically feasible.

Marine Cloud Brightening

The idea behind the marine cloud-brightening (MCB) geoengineering technique is that seeding marine stratocumulus clouds with copious quantities of roughly monodisperse sub-micrometre sea water particles might significantly enhance the cloud droplet number concentration, and thereby the cloud albedo and possibly longevity. This would produce a cooling, which general circulation model (GCM) computations suggest could—subject to satisfactory resolution of technical and scientific problems identified herein—have the capacity to balance global warming up to the carbon dioxide-doubling point. We describe herein an account of our recent research on a number of critical issues associated with MCB. This involves (i) GCM studies, which are our primary tools for evaluating globally the effectiveness of MCB, and assessing its climate impacts on rainfall amounts and distribution, and also polar sea-ice cover and thickness; (ii) high-resolution modelling of the effects of seeding on marine stratocumulus, which are required to understand the complex array of interacting processes involved in cloud brightening; (iii) microphysical modelling sensitivity studies, examining the influence of seeding amount, seed-particle salt-mass, air-mass characteristics, updraught speed and other parameters on cloud–albedo change; (iv) sea water spray-production techniques; (v) computational fluid dynamics studies of possible large-scale periodicities in Flettner rotors; and (vi) the planning of a three-stage limited-area field research experiment, with the primary objectives of technology testing and determining to what extent, if any, cloud albedo might be enhanced by seeding marine stratocumulus clouds on a spatial scale of around 100×100 km. We stress that there would be no justification for deployment of MCB unless it was clearly established that no significant adverse consequences would result. There would also need to be an international agreement firmly in favour of such action.

Submerged cylinder wave energy device: theory and experiment

Abstract Linearized water wave theory is used to show that a submerged long circular cylinder suitably constrained by springs and dampers to make small harmonic oscillations, can be extremely efficient in absorbing the energy in an incident regular wave whose crests are parallel to the axis of the cylinder. Experimental results are described which confirm the theory for small amplitude waves and which suggest that the device can still be fairly efficient in waves of moderate amplitude.

Wave power availability in the NE Atlantic

FOLLOWING Salters1 proposals for the extraction of energy from sea waves, we are working on the prediction of wave power output from devices situated at favourable coastal sites around the UK. While it is hoped that adequate spectral data from sites closer inshore will become available within the next year or so, the best present data for the North-east Atlantic are from O.W.S. India2. We describe here the power available and, on simplified assumptions, the amount that might be extracted by ducks of various diameters. (The Salter ‘duck’ is a rocking cam-shaped device which is designed to give a high efficiency of energy extraction over a wide frequency band3.) Confirmation of the validity of the important assumption of additivity of power outputs from wave components of different periods is provided in a separate paper4.

Wave Powered Desalination

One of the wave energy devices studied in the 1970s and 80s was the Edinburgh duck. Figure 1, taken by Jamie Taylor in 1976, shows a duck under test in a narrow tank.

CHAPTER 6:Engineering Ideas for Brighter Clouds

It may be possible to reduce global warming by increasing the reflectivity of marine stratocumulus clouds thereby reducing the amount of solar energy that is absorbed. Quite a small change to the reflectivity could stop further temperature rise or even produce a reversion towards pre-industrial values. This paper gives a brief account of the physics behind the Twomey effect and its application for marine cloud brightening by the release of sub-micron drops of sea water into the marine boundary layer using a fleet of mobile spray vessels. We argue that the mobility of spray vessels and the short life of spray are advantageous by allowing rapid tactical control in response to local conditions. We identify the main engineering problem as spray production, which in turn requires ultra-filtration of plankton-rich seawater. The proposed engineering solutions involving Rayleigh nozzles etched in silicon and piezo-electric excitation are illustrated with drawings. The results of a COMSOL Multiphysics simulation of drop generation are given, with nozzle diameter, drive pressure, excitation frequency and power requirement as functions of drop diameter. The predicted power requirement is higher than initially hoped for and this has led to a modified vessel design with active hydrofoils giving much lower drag than displacement hulls and turbines. The active control of hydrofoil pitch angle can be used for power generation, roll stabilizing and may also reduce hull loading similarly to the suspension systems of road vehicles. The need to identify unwanted side effects of marine cloud brightening has led to a method for using climate models to give an everywhere-to-everywhere transfer function of the effects of spray in each region on weather records at all observing stations. The technique uses individual coded modulation of the concentration of cloud-condensation nuclei separately in each of many spray regions and is based on methods used for small-signal detection in electronic systems. The first use in a climate model shows very accurate measurement of changes to a temperature record and that that marine cloud brightening can affect precipitation in both directions. Replication with other climate models will be necessary. The paper ends with tentative estimates for the cost of mass production spray vessels based on actual quotations for parts of the spray generation hardware and on the cost of Flower-class corvettes used by the Royal Navy in World War II which were built in similar numbers.

Solar Radiation Management, Cloud Albedo Enhancement

Cloud albedo enhancement is one of several possible methods of solar radiation management by which the rate of increase in world temperatures could be reduced or even reversed. It depends on a well-known phenomenon in atmospheric physics known as the Twomey effect. Twomey argued that the reflectivity of clouds is a function of the size distribution of the drops in the cloud top. In clean mid-ocean air masses, there is a shortage of the condensation nuclei necessary for initial drop formation in addition to high relative humidity. This means that the liquid water in a cloud has to be in relatively large drops. If extra nuclei could be artificially introduced, the same amount of liquid water would be shared among a larger number of smaller drops which would have a larger surface area to reflect a larger fraction of the incoming solar energy back out to space.