On the selection of working fluids for OTEC power plants

This paper analyzes the effect of three different working fluids (ammonia, propane, and freon-114) on the size of OTEC heat exchangesrs and system performance. Seven different combinations of shell-and-tube heat exchangers are considered. For each combination, a simple computer model of the OTEC power system is used to compare the three fluids. The comparison is made on the basis of A/W_net, where A is the total heat transfer area (evaporator plus condenser) and W_net is the net power output of the plant. Overall, ammonia is shown to be the best fluid (i.e. it yields the lowest value of A/W_net), although in some cases only by a small margin. The thermophysical property that gives ammonia its general superiority is its relatively high thermal conductivity. The paper also discusses heat exchanger design problems associated with liquid entrainment and boiling liquid superheat.     

The maximum power,heat demand and efficiency of a heat engine operating in steady state at less than Carnot efficiency

An imperfect, Carnot-like engine operating in steady state and receiving heat through conductance k₁ from a source at T₁ and discharging heat only through a conductance k₂ to a sink at T₄ has an efficiency at maximum net power output of $\text{η}{}_{\mathrm{m}}\text{= (}\text{?}\text{g9}\text{){1 ? √(1 ? ?(1 ?}\text{T}{}_4\text{T}{}_1$)}, where ? is a non-Carnot efficiency and $\text{? =}\text{(?k}{}_1\text{+ k}{}_2\text{)}\text{(k}{}_1\text{+ k}{}_2\text{)}$.     

Thermochemical water splitting cycles: impact of thermal burdens and kinetics

Equations are rigorously derived for evaluating the thermal efficiency of a thermochemical water splitting cycle, from which it is possible to assess the impact of each heat burden or loss separately. The equations of continuity are shown to be coupled, as a consequence, heat flow is found to be the rate determining process for the operation of a thermochemical water splitting plant. Since heat flow is rate determining, the chemical rate of reaction must be fast relative to heat flow even in the asymptotic approach to completion. The conclusion is drawn that recycling of reactants, which is required if ΔG ? 0, will probably result in an uneconomical cycle.Unlike a thermomechanical engine which can be characterized by a single parameter, the thermal efficiency η, the thermochemical engine requires two, one, η, which measures the effective use of heat and a second, τ, which measures the effective use of power. The former is defined in the conventional manner, namely the work divided by the heat. The latter is defined as the ratio of the average chemical rate for product in the reaction volume for each stage multiplied by the heat required by the cycle to split a mol of water divided by the power of the source:

$\text{τ=vR}{}^{\mathrm{\alpha }}{}_{\mathrm{i}}\text{V}{}_{\mathrm{i}}\text{Q/P=vp}{}_{\mathrm{i}}\text{Q/PδH}{}_{\mathrm{i}}\text{=1}$

ν adjusts for the stoichiometry, p_i is the power absorbed at stage i and ΔH_i is the enthalpy change in the reaction at i. To maintain a balanced plant τ must equal unity.It is, also, concluded that hybrid cycles are favoured because of the additional degree of freedom.     

Fluids and parameters optimization for a novel cogeneration system driven by low-temperature geothermal sources

A novel cogeneration system was proposed and techno-economically investigated, consisting of a low-temperature geothermally-powered organic Rankine cycle (ORC) subsystem, an intermediate heat exchanger subsystem and a heat pump subsystem. The main purpose is to identify suitable working fluids (among 27 fluids with boiling point temperature ranging from −47.69 to 47.59 °C) and optimized cycle parameters for the ORC-based power generation subsystem. The screening criteria include net power output per unit mass flow rate of hot source (P_net), the ratio of total heat transfer area to net power output (A/W_net), and electricity production cost (epc). Results show that there exists optimum evaporating temperatures maximizing the P_net value and minimizing the A/P_net and epc values. The optimum temperatures vary with different screening criteria and fluids. Optimized fluids based on each screening criteria are not the same. E170, R600 and R141b show the lowest A/W_net and epc values with averagely 3.78% lower P_net value than R236ea which presents the largest P_net value.     

A probability density function for the clearness index,with applications

This paper uses classical probability theory to derive expressions for the expected (or mean) value of quantities such as the irradiation on inclined surfaces, collector output, and net gain through windows. The random variables are the clearness index k_t and the diffuse fraction k, whose means are $\text{k}{}_{\mathrm{t}}$ and $\text{k}$, respectively. The probability function for k_t is assumed to depend only upon $\text{k}{}_{\mathrm{t}}$ and is represented by a simple function chosen so that its corresponding distribution function is a close fit to the one originally graphed by Liu and Jordan. The probability function for k is represented, for a given k_t, by an impulse function centred at the $\text{k}$ observed at that k_t; the Orgill-Hollands functional form is used for the function $\text{k}\text{(k}{}_{\mathrm{t}}\text{)}$. A general purpose integral expression is presented for the expected value of any quantity which can be expressed as a functionof k, k_t and time of day. The integral is evaluated for several such quantities: the solar irradiation on an inclined surface, the output of both flat plate and concentrating collectors, and the net heat gain through a window with an automatic shutter. Example calculations are included.     

A model for the economic analysis of cogeneration plants with fixed and variable output

A method is presented for an economic sensitivity analysis of a cogeneration plant operating at off-design conditions. The economic parameters are the annual fixed costs, operating costs, fuel costs and the market value of the electrical power and capacity as paid by the local utility. The thermodynamic parameters are the required time-dependent steam energy rate [$\text{q}\text{?}{}_{\mathrm{s}}\text{(θ)}$], the plant power-to-steam energy rate ratio (PSR), and the steam efficiency (η_s). The last two parameters are shown to be functionally related to $\text{q}\text{?}{}_{\mathrm{s}}$ (which may be constant or vary with time). A numerical example is given.     

Turbulent convective transfer in plate heat exchangers

Readily available data on turbulent transfer in plate heat exchangers can be correlated by a heat transfer-energy dissipation analogy:

$\text{Nu}\text{g}{}_1\text{(pr, Vi)}\text{=C}{}_3\text{(fRe}{}_3\text{)}{}_{\mathrm{\delta }}$

in which the Nusselt number modified for changes in the Prandtl number and bulk to wall viscosity ratio Vi is related to the friction factor f and the Reynolds number. The exponent e is a weak function of the coefficient C₃ which depends on the corrugation geometry.When using chevron or herringbone type patterns the heat transfer depends significantly on the angle between the plate corrugation and the main flow direction. If this angle is π/4 the heat transfer per unit of mechanical energy dissipated is a maximum. Although maximum transfer (with maximum pressure drop) is obtained at π/2, a more practical angle giving high transfer at moderate pressure drops in 2π/5.     

Calculation of heat transfer coefficients for nucleate boiling in binary mixtures of refrigerant-oil

The heat transfer coefficient for nucleate boiling of pure liquids can be determined in many cases by the simple relation $\text{h = C · q}{}^{\mathrm{n}}$. In nucleate boiling of mixtures with widely varying properties, the concentration gradient close to the heating surface strongly affects the heat transfer. As the composition of the mixture is difficult to obtain there, it is tried to develop relations as simple as the one mentioned above. The following form is chosen $\text{h = C (Y) · q}{}^{\mathrm{n(Y)}}$ with Y being a function of both, the kind of mixture A_K and the concentration w: $\text{Y}\text{= f (}\text{A}{}_{\mathrm{K}}\text{, w}\text{)}$. Based on experimental values for four different refrigerant-oil mixtures in concentrations of $\text{w = 0.005}\text{to}\text{0.20}$, the following relation renders best results: $\text{h}\text{= 0.085·[}\text{exp. (b}{}_1\text{w}\text{) +}\text{exp.(b}{}_2\text{w}\text{)],}\text{q}\text{(0.89-}\text{Bw}\text{)}$ For each kind of oil, however, different values of b₁, b₂ and B have to be used; these are given.     

Droplet evaporation: Effects of transients and variable properties

A numerical study of the effects of transients and variable properties on single droplet evaporation into an infinite stagnant gas is presented. Sample calculations are reported for octane droplets, initially at 300°K with R_o = 0·1, 0·5, 2·5 × 10^?4m, evaporating into air at temperatures and pressures in the ranges 600–2000°K and 1–10 atm, respectively. It is found that initial size R_o is eliminated from the problem on scaling time with respect to R²₀ and that the evaporative process becomes quasi-steady with ($\text{R}\text{R}{}_0\text{)}{}^2\text{= (}\text{R1}{}_0\text{R}{}_0\text{)}{}^2\text{?}\text{βt}\text{R}{}^2{}_0$, as suggested by experiment. Comparisons of solutions using various reference property schemes with those for variable properties show that best agreement obtains using a simple $\text{1}\text{3}$ rule wherein properties are evaluated at $\text{T}{}_{\mathrm{r}}\text{= T}{}_{\mathrm{s}}\text{+}\text{(T}{}_{\mathrm{e}}\text{?T}{}_{\mathrm{s}}\text{)}\text{3}$ and $\text{m}{}_{\mathrm{1,r}}\text{= m}{}_{\mathrm{1,s}}\text{+}\text{(m}{}_{\mathrm{1,e}}\text{? m}{}_{\mathrm{1,s}}\text{)}\text{3}$. The effects of temporal storage of mass species, energy, etc. and radial pressure variations in the vapor phase prove to be negligible, the early transient behavior being solely due to sensible heat effects within the droplet and related variations in vapor-side driving forces.     

Surface renewal theory for turbulent mixed convection in vertical ducts

A heat transfer correlation for opposing mixed turbulent convection in vertical ducts was obtained utilizing surface renewal theory. The correlation was found to be Nu_Db = 0.0115Re_Dh^0.8Pr^0.5 The correlation fit data to within 7% over a parameter range of 0.7 < Pr < 7, 1 × 10⁴ < Re_Dh < 2 × 10⁴, and 1 × 10⁶ < Gr_Dh < 2 × 10⁹. The mean residence time, characterizing the time a clump of fluid resides on the wall, was found to decrease as both and Re_Dh increase. This explains the enhanced heat transfer due to buoyancy in opposing mixed turbulent flows. This heat transfer enhancement was also reflected in a decreasing thermal boundary layer thickness with increasing Re_Dh, Gr_Dh or Pr.     

Water splitting reaction on a polynaphthoquinone catalyst a polynaphthoquinone-SO2-I2 system for H2O decomposition

The water splitting reaction in a polynaphthoquinone-SO₂-I₂ system under mild conditions is reported. One mole H₂O was decomposed to form 2 moles HI and 1 mole H₂SO₄ by the following two successive hydrogen transfer reactions in conjunction with the redox cycle of quinones(Q)-hydroquinones(QH₂) in the polynaphthoquinone catalyst:

$\text{2H}{}_2\text{O +SO}{}_2\text{+ Q ? QH}{}_2\text{+ H}{}_2\text{SO}{}_4\text{, QH}{}_2\text{+ I}{}_2\text{? 2HI + Q}$

The hydrogen transfer reaction from H₂O to Q was accelerated by a factor of 3–5 by light irradiation and hence this system could work also as a hybrid thermochemical cycle. When the catalytic hydrogen transfer reactions on the polynaphthoquinone are combined with the thermal decompositions of HI(2 HI ? H₂ + I₂) and H₂SO₄ ($\text{H}{}_2\text{SO}{}_4\text{? H}{}_2\text{O + SO}{}_2\text{+}\text{1}\text{2}\text{O}{}_2$), a closed water splitting cycle for the hydrogen production could be constructed. The reaction mechanism is also discussed.     

Linear stability analysis of double-diffusive solar pond

In this communication, the stability of the double-diffusive solar ponds has been investigated in the linear approximation. The corresponding linearized system of equations of motion is reduced to a single integro-differential equation using the Green-function technique. In contrast to the conclusions of Veronis that, initially, the instability occurs as an oscillatory mode and at a value of R_T (Rayleigh number for temperature) greater than R_S the motion becomes steady, the present analysis shows that, initially, as R_T increases from zero but remains considerably less than R_S, exponentially growing and decaying modes (steady motion) occur first; for a value of R_T more than a critical value R_T^c, the motion becomes oscillatory. This oscillatory motion may, due to the basic non-linear dynamics of the system, even become aperiodic. Further, for R_S → ∞, the minimum value of R_T for which steady motions can occur tends to $\text{K}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>2</mtext>}}\text{·R}{}_{\mathrm{S}}$, where $\text{K}\text{= K}{}_{\mathrm{S}}\text{/K}{}_{\mathrm{T}}$ where K_S and K_T are diffusivity coefficients for salt and temperature, respectively; as a contrast, according to Veronis, $\text{R}{}_{\mathrm{T}}\text{a}\text{?}\text{σ}{}^{\mathrm{?1}}\text{R}{}_{\mathrm{S}}\text{; σ = v/K}{}_{\mathrm{T}}\text{, v}$ being the kinematic viscosity.     

Thermal insulation provided by plain,horizontal annular cavities containing atmospheric pressure air

For the range 3 × 10³ ≤ Gr_di ≤ 10⁸ and $\text{1·3 ≤}\text{r}{}_0\text{r}{}_{\mathrm{<mtext>i</mtext>}}\text{≤ 7·5}$, it is suggested that for the steady-state rate of heat transfer outwards by combined laminar, free convection and conduction through the atmospheric pressure air contained within horizontal concentric annuli. This simple correlation, evolved from an analysis of published, as well as new, experimental information, will enable designers to predict the combined convective/conductive resistance provided by the contained air for the range of concentric pipes likely to be encountered in practice.An optimal eccentricity of 0·24 (the inner cylinder being moved vertically upwards relative to the outer cylinder from the concentric position) corresponds to the maximum combined convective/conductive resistance configuration. For the systems tested in the temperature range 18°C ≤ T ≤ 150°C, this optimal eccentricity is not significantly affected by changes in the surface emissivities.     

Influence de la variation de la viscosite avec la temperature sur le frottement avec transfert de chaleur en regime turbulent etabli

Experimental research about measurement of local friction pressure drop in fully developed turbulent flow of water in a smooth and uniformly heated circular tube has been investigated.The effect of variable viscosity with temperature in the laminar boundary layer is isolated by varying the heat flux at constant values of the local bulk Reynolds and Prandtl numbers. This severe method shows that the usual dependence of friction ratio $\text{f}{}_{\mathrm{H}}\text{f}{}_{\mathrm{iso}}$ on viscosity ratio $\text{μ}{}_{\mathrm{p}}\text{μ}{}_{\mathrm{b}}$ is more complex than the simple power law appearing in the familiar empirical results. It has been found that the exponent is a complicated function of Reynolds number and the viscosity ratio and is independent of the Prandtl number in our experimental range.We propose a new relationship for the friction coefficient ratio in which the viscosity ratio is changed by the more accurate parameter: $\text{x}\text{\$}\text{?}\text{= (-μ}{}_{\mathrm{b}}\text{-1/μ}{}_{\mathrm{p}}\text{)(μ}{}_{\mathrm{b}}\text{/μ}{}_{\mathrm{b}}\text{/μ}{}_{\mathrm{p}}\text{)}{}^{0.17}$ which is directly proportional to the heat flux when the Reynolds and the Prandtl numbers are fixed. The dispersion is caracterised by a standard deviation σ = ±0·7%, 98% of the points are within 2σ = ±1·4%. The agreement between prediction and the previously existing results can be considered satisfactory for Reynolds number from 2 × 10⁴ to 30 × 104 and Prandtl number from 2 to 6.     

Optimizing energy transition paths in CO2 emission reduction strategies

In the consideration of energy use scenarios designed to avoid excessive build-up of atmospheric carbon dioxide and the effects of the associated climatic changes it can be anticipated that difficulties will ensue if needed rates of increased deployment of non-fossil fuel burning energy plants become very large. This may result if allowable maximum CO₂ concentrations are set sufficiently low, or if acceleration of non-fossil fuel energy use is sufficiently delayed.In a broad ranging numerical investigation of this problem, Perry et al.¹ have suggested that difficulty will be experienced in the transition to a non-fossil based economy, not only if the rate of growth of non-fossil fuel energy use rate, $\text{E}\text{?}{}_{\mathrm{N}}$ is excessive, but also if the second derivative, $\text{E}\text{?}{}_{\mathrm{N}}$, becomes too large. This observation has important ramifications, since $\text{E}\text{?}{}_{\mathrm{N}}$ may exceed tolerable limits, whilst $\text{E}\text{?}{}_{\mathrm{N}}$ still remain modest in size. In making the estimates an empirical method for joining early and asymptotic energy use rates was employed. In order to make sure that the conclusions reached were not sensitive to the ad hoc nature of this technique, we have introduced a more rigorous approach by using a variational interpolation approach that minimizes the peak value of $\text{E}\text{?}{}_{\mathrm{N}}$ (or a combination of $\text{E}\text{?}{}_{\mathrm{N}}$ and $\text{E}\text{?}{}_{\mathrm{N}}$.$\text{E}\text{?}{}_{\mathrm{N}}$ We find that such an optimization procedure can reduce the maximum values of $\text{E}\text{?}{}_{\mathrm{N}}$ by as much as 50% from those calculated by Perry et al. This indicates that the energy transition process should be easier that concluded by them, at least as far as a criterion based upon $\text{E}\text{?}{}_{\mathrm{N}}$ variation is concerned, but can still create difficulties, especially if a 500 ppmv CO₂ ceiling is considered hazardous and if actions to reduce fossil fuel use are delayed too long.     

The heat storageloss ratio for a building and its response time

If a thermal system is in a steady state and at zero time a step change in excitation is applied, the time taken for the temperature at some node to change by 63 per cent of its ultimate change is defined as the response time, t_r, there. If the temperature at the node concerned is fixed, the ratio of the heat, S, stored in the system relative to an ambient temperature of zero, to the heat, L, lost from the system, provides another time, $\text{t}{}_{\mathrm{sl}}\text{=}\text{S}\text{L}$. For many years, t_sl, which is easy to calculate, has been assumed to be of order t_r, which is easy to interpret. When this is so, the $\text{storate}\text{loss}$ time provides a measure of the speed of the building's thermal response.Previous work has demonstrated that, in some circumstances, t_sl may equal t_r exactly, or almost exactly, but that, in other circumstances, the two times may differ considerably. This paper examines the relationship between the two for a number of elementary thermal systems whose response can be calculated using published solutions. The effects corresponding to changes in ambient temperature and to a change in heat input have been examined.It appears that, whilst $\text{t}{}_{\mathrm{r}}\text{t}{}_{\mathrm{sl}}$ can vary over a wide range, for the most part the ratio lies fairly close to unity and it seems likely that this may be the case for a building enclosure with its complex pattern of heat exchange and storage.It is useful to introduce a third time, the fundamental decay time, t_d. The value of t_r is of the same order of magnitude as t_d, or is less than it.A standing wave matrix, similar in structure to the transmission matrix for handling progressive waves in a slab, is used to find the system's eigenfunctions.A review is given of some previous work concerned with the relationship between building response time and the $\text{storage}\text{loss}$ ratio.     

Thermochemical hydrogen preparation—Part V. A feasibility study of the sulfur iodine cycle

A sulfur-iodine cycle consists of the following three reactions:

$\text{2H}{}_2\text{O + SO}{}_2\text{+ I}{}_2\text{→ H}{}_2\text{SO}{}_4\text{+ 2HI,}$

$\text{H}{}_2\text{SO}{}_4\text{→ H}{}_2\text{O + SO}{}_2\text{+}\text{1}\text{2O}{}_2\text{,}$

$\text{2 HI → H}{}_2\text{+ I}{}_2\text{.}$

It was found that the first reaction can be performed as a cell reaction without the addition of external energy. The sulfuric acid and the hydriodic acid are produced separately in the anode and cathode compartments, respectively. The second and third reactions can be carried out as catalytic thermal decompositions. A process flow sheet of this cycle and its mass balance was based on experimental results, and the heat balance for this cycle was made. It was found that internal heat exchange for this cycle was very large (about 2600 kcal/mol H₂), due mainly to the low yield of the decomposition reaction of hydrogen iodide. Theoretical and experimental studies were made to improve the yield of this reaction. The following three methods seem to be promising for this purpose: (1) continuous removal of the hydrogen produced in the reaction zone; (2) performance of the reaction at low temperature (185–250°C) and high pressure (100 atml; and (3) substitution of the benzene-cyclohexane cycle (6HI ? C₆H₆ → C₆H₁₂ + 3I₂; C₆H₁₂ → C₆H₆ ? 3H₂) for the hydrogen iodide decomposition step.