Pertti Kauranen

A multicomponent PCM wall optimized for passive solar heating

Abstract The use of phase change materials (PCMs) for short-term heat storage in direct-gain passive solar applications is discussed. Approximate formulae are presented for optimum phase change temperature and the thickness of a PCM wall. Numerical simulations based on the Test Meteorological Years of Helsinki, Finland (60°N) and Madison, Wisconsin (43°N) indicate that a phase change temperature of 1–3°C above the average room temperature would yield optimal diurnal heat storage results. A desired phase change point can be accurately obtained by using fatty acids and their mixtures. To ease the installation, PCMs can be impregnated into conventional construction materials such as plasterboard. The thermal performance of a PCM wall in the direct-gain room in a residential application was briefly studied through hourly simulations. According to conservative estimates, direct energy savings of 5–20% could be expected, depending on climate. As this may not always be adequate for economic cost-effectiveness, the effect of increased thermal comfort plays also a key role in evaluating the total benefits of PCMs storage.

Kinetics of methanol oxidation on carbon-supported Pt and Pt + Ru catalysts

The kinetics of methanol electro-oxidation on carbon-supported particulate Pt and Pt + Ru catalysts is studied on PTFE-bonded electrodes made of these materials. The electrochemical characterization is carried out both galvanostatically and potentiostatically in sulphuric acid, and special attention is paid to the effects of low methanol concentrations in the range from 5.0 mM to 2.0 M. Furthermore, the effects of reaction temperature, catalyst loading and electrolyte concentration are discussed. A steady-state adsorption model which explains the observed polarization behaviour is presented. The model highlights the subtle balance between the initial methanol adsorption-dehydrogenation and the subsequent oxidative removal steps as well as the important roles of water and surface intermediates in the oxidation reaction. Rough estimates are given for the rate constants and activation energies of the individual reaction steps.

Methanol permeability in perfluorosulfonate proton exchange membranes at elevated temperatures

A simple electrochemical method for the measurement of the permeability of methanol in proton exchange membranes equilibrated with a supporting liquid electrolyte at elevated temperatures is proposed. Carbon supported platinum working electrodes are placed to both sides of the membrane sample and serve as concentration sensors. Methanol is added to one or both sides of the membrane and the permeability is calculated from the time responses of anodic peak currents on the two working electrodes. Experimental results are given for Nafion® 117 perfluorosulfonate membrane in 2.Om H2SO4 at 60 and 70°C.

An organic PCM storage system with adjustable melting temperature

Abstract A proper storage temperature is an important criterion for selecting a phase change material (PCM) for a passive solar heating application. Here we describe a novel procedure to produce a mixture of carboxylic acids with a melting temperature adjustable to the climate specific requirements. The approach is based on the ideal solution model and differential scanning calorimetry (DSC). The applicability of the method is demonstrated and it is also applied to a PCM wall design. The accuracy of the theoretical model is ±2°C in the temperature range of 20°–30°C and even a ±0.5°C accuracy can be obtained by the experimental procedure.

Mixed methanol oxidation/oxygen reduction currents on a carbon supported Pt catalyst

The presence of methanol at the cathode may cause depolarisation of the Pt cathode used for oxygen reduction in a direct oxidation methanol fuel cell. A semi-empirical model is presented to describe the simultaneous oxygen reduction and methanol oxidation processes on a carbon supported Pt gas diffusion electrode used in practical fuel cells. The model is verified against potentiostatic polarisation data measured at 80°C in sulphuric acid over a wide range of methanol concentrations using both oxygen and air as the oxidative reactant. The results confirm earlier observations that the mixed net current is formed as a sum of the anodic methanol oxidation and the cathodic oxygen reduction current. Of these, the methanol oxidation current is very little affected by the presence of oxygen, but the oxygen reduction current is suppressed by surface intermediates from the methanol oxidation process.

Water and methanol uptake in proton conducting Nafion® membranes

Methanol uptake from methanol–water mixtures has been measured at ambient temperature in Nafion®117 membranes initially saturated with water. After equilibration for 18 h the membranes were dried at 70°C in vacuum with a cold trap cooled with liquid nitrogen and the total amount of absorbent was determined from the weight loss. The methanol–water ratio was determined by 1H NMR. A plot of the mole fraction of methanol in the membrane vs. the mole fraction in the water–methanol mixture was linear in the whole range from pure water to pure methanol. The slope indicated that Nafion® does not preferentially take up either water or methanol. The number of solvent molecules was found to be 23 per sulphonic group for both methanol and water.

Development of a self-sufficient solar-hydrogen energy system

Abstract This paper describes the status of a photovoltaic hydrogen energy system development project at Helsinki University of Technology at the end of June 1992. The objective of the project is to demonstrate the technical feasibility of a 100% self-sufficient energy system based on solar photovoltaics (PV) and hydrogen technology. Basically, PV electricity is used to produce electrolytic hydrogen, which is stored over the season to be converted back to electricity in a fuel cell. The pilot plant has been designed for a 1–2 kWh day−1 constant electric load in the climate of Helsinki (60°N). The work so far has included component and subsystem testing, as well as optimization of the total system and its control through comprehensive numerical modelling. Experimental results are given for the electrolyser performance as well as for a 1 month operation of the hydrogen production subsystem. The numerical simulation shows excellent agreement with measurements and is used to predict the pilot plant performance over a 1 year time period.

Operation experiences of a phosphoric acid fuel cell in a solar hydrogen energy system

A phosphoric acid fuel cell (PAFC) has been connected to a small-scale autonomous solar hydrogen energy system. The performance of the fuel cell has been studied by using its current-voltage curves and energy balance calculations. According to the operation experiences, the advantages of the PAFC are its ability to use air as the oxidant, compact design without electrolyte loop, and high value waste heat. On the other hand, the disadvantages are the need of pre-heating and the open-end stack construction which cause significant energetic losses and decrease the operational efficiency.

Simulation of solar hydrogen energy systems

Abstract Solar hydrogen is a promising long-term global energy option for the post-fossil fuel era. On the other hand, solar hydrogen may have already found an early commercial application in the form of seasonal energy storage for remote stand-alone photovoltaic (PV) applications. In a stand-alone solar hydrogen energy system, the photovoltaic array is coupled with an electrolyser to produce H2 which is stored to be later converted back to electricity in a fuel cell. The system setup comprises several subsystems which have to be controlled in an optimal way. Numerical simulations are used to get a closer insight into the transient response behavior of these elegant, but rather complicated systems during variable insolation conditions and to estimate the overall system performance accurately over extensive periods of time. The simulations are performed with the H2PHOTO program which has been successfully used for the design of a solar hydrogen pilot plant. It has also shown good accuracy against experimental data.

Control of battery backed photovoltaic hydrogen production

Abstract A battery backed photovoltaic (PV) hydrogen production system shows several advantages over non-battery systems, especially in cases in which the PV array is not totally dedicated to hydrogen production. The use of battery state-of-charge (SOC) and time of the day limits as control parameters to optimize hydrogen production in a battery backed PV-electrolysis system may increase the annual H2 production by more than 10%. Through shunt resistor regulation of the electrolyzer current, a high system efficiency may be maintained even if the power of the PV array well exceeds that of the electrolyzer.