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Project Descriptions

Next Generation Solar Heating Systems: Low Cost Polymer-based Systems

Over the past five years, the University of Minnesota team has collaborated with NREL, E.I. DuPont de Nemours, Solvay Advanced Polymers, FAFCO, and Davis Energy Group to develop low-cost polymer based solar water heating systems for mild climates. The work to date has focused on material and thermal performance issues related to unpressurized integral collector storage systems that use a load-side immersed heat exchanger. The University of Minnesota team has addressed issues surrounding the immersed heat exchanger. We have developed material selection criteria, tested candidate materials for compatibility and durability in hot chlorinated water, evaluated the build up of calcium carbonate (scale) on candidate polymeric materials, developed empirical models and design tools for evaluating thermal performance of heat exchangers immersed in shallow collectors and developed testing protocols and tested prototype heat exchangers submitted by industry. Recently, we have investigated methods to improve thermal performance by promoting stratification with the collector. Research addresses fundamental issues that have been identified as either barriers to implementation or offer the opportunity of substantial improvements in performance or cost reduction. Work is underway to consider the next generation systems that offer opportunities for greater market penetration.

Optimization of Polymer ICS Systems
The University of Minnesota has previously conducted extensive experiments on unpressurized ICS systems that incorporate top-mounted tube bundle heat exchangers and has an experimental facility and instrumentation dedicated to this research. The system concept is shown below. During thermal charging, the fluid in the ICS is heated by incident solar radiation and becomes thermally stratified. During discharge, water flowing through the tubes is heated by natural convection and any existing stratification is quickly destroyed. Location of the heat exchanger in the top of the enclosure has two advantages: first, energy is withdrawn from the entire storage volume since dead spots are essentially eliminated and second, the fluid circulation is strongest because the elevation difference between the immersed heat exchanger and the enclosure bottom is largest. The increased strength of the fluid circulation translates into larger natural convection heat transfer coefficients.

Unpressurized polymer-based integral solar collector storage (ICS) system with an immersed tube bundle located in the top portion of the collector storage.

The overall objective of ongoing research is to develop and evaluate innovative methods of passively controlling the flow processes in the storage compartment to maintain desirable hot water outlet temperatures. This work has the potential to reduce overall system cost and increase system efficiency. Experimental and computational fluid dynamic modeling of new systems is underway. One concept under investigation is the use of a divided storage. Results to date indicate that simply dividing the storage into two compartments can increase energy delivered by more than 10 percent.

Heat Exchanger Testing
The University of Minnesota continues to work closely with industry to characterize industry heat exchanger concepts and prototypes. The Thermal Characterization Laboratory includes sophisticated equipment to measure heat transfer rates in a variety of heat exchanger configurations.

Heat Exchanger Materials Durability
The key to selecting polymer materials and designing components for solar hot water applications is the long term mechanical performance of the polymer in a hot potable water environment. For structural components (such as the heat exchanger), the strength and stiffness after 10 years (or more) exposure to hot potable water must be predicted. Potable water is characterized by its oxidation reduction potential (ORP). As part of a potable hot water system, polymer components can be exposed to temperatures as high as 82°C and ORP levels as high as 500 mV. The University of Minnesota is conducting experiments to characterize the mechanical behavior of polymers for solar hot water applications. Mechanical properties such as creep compliance, creep rupture strength and ultimate tensile strength are measured. Tests are conducted in a water bath under controlled temperature and ORP conditions.

Scaling of Polymers
The formation of calcium carbonate scale on polymers is of great importance to the use of polymeric materials in potable water pipes and heat exchangers. In this project, experiments are in progress to help understand the mechanism by which polymers scale. The goal is to develop strategies to avoid scaling. Of key interest are chemical differences between polymers, which influence their interaction with water, the mechanism of calcium carbonate nucleation, and scale morphology, and structural differences (e.g., surface roughness), which may impact calcium carbonate nucleation and adhesion.

Scale on nylon 6,6.

Energy Efficient Roof/Attic

The University of Minnesota and Pulte Home Sciences are collaborating to develop an innovative residential roof with the primary objective of creating a more energy efficient building envelope. The Department of Energy reports that roofs represent 16% of the heating energy use and 14% of the cooling energy use in post-2000 new construction. The goal is to design, build and evaluate a modular roof panel manufactured in a continuous process. The roof panel will be self-supporting, have an effective R-value at least 20% greater than that required by code. Second generation models are planned to incorporate energy saving or production technologies such as heat recovery, photovoltaics and/or solar hot water collectors. This innovative approach to roof construction will eliminate the need for additional roof support (trusses or joists) and provide conditioned space for HVAC equipment and storage in the attic (usually termed a cathedralized attic). The project involves an integration of design, manufacturing and marketing.

As a result of this study, “best” concepts have been selected for continued study and development, including prototype construction and testing. The best design concepts were selected based on their superior rating in terms of total weight and depth required to provide adequate structural support and the desired thermal performance, cost per unit area of roof surface, suitability for cost effective manufacture, reduction of waste, ease of construction, and builder acceptance. The symmetric sandwich panel, which appeared at first to be a very promising concept because of its simplicity and well established use in commercial buildings and SIP walls, has a longevity limitation as a self-supporting roof component because of core creep. There is concern regarding continuous, long term loading of polymer foam materials.

Solar Production of Hydrogen with a Two-step Water-splitting Thermochemical Cycle

in collaboration with
Professor Aldo Steinfeld
ETH-Swiss Federal Institute of Technology

An efficient technology for solar hydrogen production via a two-step water-splitting thermochemical cycle based on zinc oxide/zinc redox reactions will be demonstrated and optimized. The project is a collaboration of the University of Minnesota’s Department of Mechanical Engineering and the ETH- Swiss Federal Institute of Technology. The first step of the process is dissociation of zinc oxide to zinc and oxygen. This reaction is endothermic and can be carried out in a reactor heated by concentrated solar energy. The second step is hydrolysis of zinc to produce hydrogen and zinc oxide, which can be recycled in the solar reactor. The net reaction is the splitting of water. This project focuses on a novel combined process for the hydrolysis step, which has been identified as one of the major challenges to implementation. The process encompasses the formation of zinc nanoparticles followed by their in-situ hydrolysis for hydrogen generation. The advantages of using nanoparticles are three-fold: 1) their high specific surface area augments the reaction kinetics, heat transfer, and mass transfer; 2) their large surface to volume ratio favors complete or nearly complete oxidation; and 3) their entrainment in a gas flow allows for continuous and controllable feeding of reactants and removal of products. The experimental study and modeling work to be undertaken will address the important scientific and technical challenges of optimizing the combined nanoparticle formation and hydrolysis process. The results will lead to development and scale-up of a chemical reactor that optimizes the production of hydrogen in a truly sustainable process with zero greenhouse gas emissions.

Hydrogen represents a potentially important clean energy carrier, but today, most industrial hydrogen is made using fossil fuel (natural gas) as the raw material as well as the energy source for steam reformation. The use of concentrated solar energy to carry out high temperature processes that produce hydrogen from water provides a truly sustainable alternative with zero greenhouse gas emissions. In addition, the conversion of solar energy into stored chemical energy overcomes the drawback of using a diluted and intermittent energy source. In very simple terms, the conversion of solar energy into solar fuels (in this case hydrogen) permits the storage and transport of the earth’s most abundant energy source.

Several routes for the thermo-chemical production of hydrogen from water using solar energy have been examined [1-4]. Direct thermal dissociation of water, although conceptually simple, is not feasible. It is impeded by the need for a high-temperature heat source to achieve a reasonable degree of dissociation, and by the need to separate H2 and O2 at high temperature to avoid recombination and explosive mixtures. On the other hand, as illustrated in Fig. 1, two-step metal oxide redox reactions produce the same net result – the dissociation of water – at lower temperature and without the need to separate H2 and O2.

The first step is dissociation of a metal oxide (MxOy) to the metal (M) and oxygen:
(1) This reaction is endothermic and can be carried out in a reactor heated by concentrated solar energy. The second step is hydrolysis of the metal to produce hydrogen and the corresponding metal oxide:
(2) This reaction is exothermic and can be carried out on-site where the hydrogen is to be used. The metal oxide formed in (2) can be recycled in the solar reactor via reaction (1). The net reaction is the splitting of water:
(3) Numerous metals have been considered for this process (e.g., B, Fe, Ti, Co, Ni, Sn, and Zn). The metal-oxide/metal pair ZnO/Zn is favored for many reasons. First, at 2235K, the Gibbs free energy for the dissociation reaction,
(4) is zero. Thus, the endothermic reaction can proceed using only solar thermal energy. Second, reaction (4) is more easily achieved in a solar concentrator than the dissociation of other metal oxides that require temperatures above 3500 K. Third, the exergy efficiency of the cycle is high. The theoretical upper limit is 44% with complete heat recovery, and 29% without any heat recovery [5]. Fourth, in comparison to production of H2 by steam reforming of natural gas, the proposed cycle generates high-purity H2, required in PEM fuel cells, because it is the only gaseous product of the hydrolysis reaction (5):
(5) There are additional ladvantages of using Zn for on-site production of H2. For example, Zn can be transported to the H2 delivery site for on-site generation, bypassing the complications associated with the long-range transport and storage of H2.

Most of the work-to-date on solar production of H2 via the ZnO/Zn redox reactions has focused on the solar step. Chemical aspects of the thermal dissociation of ZnO have been investigated [6-8], exploratory tests have been carried out in solar furnaces [9-11], and a solar chemical reactor has been designed and experimentally demonstrated at a power level of 5 kW [12]. Effective separation of the gaseous products to avoid reoxidation of Zn is needed. Possible approaches are a rapid quench or electrolytic separation at high temperatures.

The second-step hydrolysis reaction has its own challenges. The reaction is thermodynamically favorable at or below about 1490K, where the Zn phase can either be solid, liquid or gas. Prior work shows that hydrolysis of Zn is constrained because of the formation of a protective coating of ZnO at the reactant surface. In experiments in which steam was bubbled through molten zinc at 723 to 773K, the production of H2 was limited by the formation of a protective ZnO(s) layer around the steam bubbles [13]. Similarly, passivation of Zn(s) particles has been observed in recent work by the co-PIs. An intriguing solution to this problem was recently proposed and tested in very preliminary experiments at ETH [13,14]. The novel combined process for the efficient execution of the 2nd step, reaction (5) encompasses the formation of Zn-nanoparticles followed by their in-situ hydrolysis for H2 generation. The advantages of using nanoparticles are 3-fold: 1) their inherent high specific surface area augments the reaction kinetics, heat transfer, and mass transfer; 2) their large surface to volume ratio favors complete or nearly complete oxidation; and 3) their formation and entrainment in a gas flow allows for continuous feeding of reactants and removal of products. The nanoparticles can be produced by evaporation-condensation processes.

The scientific and technical challenges of the combined nanoparticle formation and hydrolysis processes lie in understanding the kinetics of the combined reactions and in developing a chemical reactor that optimizes the production of hydrogen and at the same time facilitates the removal of ZnO, which in preliminary experiments has been found to coat the reactor wall. The research to be conducted at the University of Minnesota complements the on-going experimental and modeling work at ETH.

Experiments on the combined nanoparticle formation followed by in-situ production of H2 via hydrolysis will be conducted in an effort to determine reaction rates and to optimize the production of hydrogen as well as facilitate removal of the product ZnO. In addition, because the energy efficiency of the process depends strongly on the ability to use the heat of the hydrolysis reaction to evaporate Zn and H2O, development of a heat exchange mechanism will be considered.

The project is divided into five tasks. During year 1, the goal of Task 1 will be to design, fabricate and calibrate a chemical reactor for the combined process. It is anticipated that the reactor will feature three zones: 1) a mixing zone where separate streams of Zn vapor and steam will be mixed efficiently; 2) a nanoparticle formation zone, where Zn vapor will be steam-quenched below its saturation temperature to form nanoparticles, and 3) a hydrolysis reaction zone, where Zn nanoparticles will react with steam to generate H2 and ZnO. Such an arrangement will allow the continuous feeding of the reactants under controlled stoichiometry,and the continuous removal of products. The high specific area of the nanoparticles is expected to enhance the reaction kinetics and heat/mass transfer, which will in turn permit short residence times (fraction of a second) and, consequently, small reactor volumes.

The goal of Task II, also conducted during the first year, will be to develop the peripheral equipment to provide a controlled feed of reactants (Zn-evaporator, steam generator, etc.), temperature control, off-gas handling, and instrumentation to monitor temperature, pressure, flow rate as well as analyses of the composition of both gas and solid phases. The measurement and data acquisition system will include temperature, pressure, and mass flow rates measurement devices, experimental control and safety devices, and instrumentation needed for gas/solid composition analysis. (The gas chromatograph in Professor Davidson’s laboratory will be modified for this purpose.)
During years 2 and 3, experiments to characterize the mechanisms of nanoparticle oxidation, hydrogen yield and recovery of ZnO will be conducted. During the second year in Task III, initial experiments will be conducted to demonstrate the technical feasibility of the process and to characterize reactor performance. The reactor’s performance will be determined from mass and energy balances for experimental runs of selected temperatures, mass flow rates, and stoichiometry. It will require the execution of the following steps:

1) measurement of mass flow rates and temperatures of reactants and products;
2) chemical analysis of reactants and products (gases and solids);
3) determination of the extent of reaction based on step 2;
4) calculation of the enthalpy change of the reaction based on steps 1-3;
5) calculation of thermal efficiency based on steps 1-4.

The H2 yield will be determined with respect to the evaporated Zn as:

(6) where H2,produced refers to the amount of H2 at the reactor outlet (measured by gas chromatography), and H2,max refers to the theoretical maximum amount of H2 that could have been produced under complete hydrolysis of the evaporated zinc. The conversion of Zn will be determined by the composition of the particles collected downstream (measured by XRD) as:

(7) At the end of Task III, the technical feasibility of the process will have been experimentally demonstrated using the reactor prototype.

During the third year in Task IV, the effects of temperature, residence time, and H2O:Zn molar ratio will be evaluated in parametric experiments. The objective will be to optimize the yield of hydrogen. The influence of the operating conditions on the thermal performance and on the quality of the reaction products will be examined. The challenge will be to find the reactor/process configuration for matching the rate of nanoparticle formation to the rate of the hydrolysis reaction. These results will be used to develop design and operating guidelines for optimized reactors and to validate the modeling efforts in Task V.

The objective of Task V is to support modeling efforts at ETH. Formulation of a model that couples the fluid and particle dynamics with the chemical reaction kinetics will guide the design of reactors and reactor processes. The model will account for nucleation, condensation, coagulation, surface reactions, and sintering. The continuity, momentum, and energy governing equations for two-phase reacting flow will be coupled to the chemical reaction kinetics and to the particle dynamic models. The experimental data will be used to validate the model.

REFERENCES
[1] Fletcher, E.A., Moen, R.L. Hydrogen and Oxygen from Water. Science 1977; 197:1050-1056.
[2] Fletcher, E.A. Solarthermal processing: A review. J. of Solar Energy Engineering 2001; 123:63-74.
[3] Steinfeld A. Solar Thermochemical Production of Hydrogen - A Review. Solar Energy 2004, in press.
[4] Steinfeld A, Kuhn P, Reller A, Palumbo R, Murray J, Tamaura Y. Solar-Processed Metals as Clean Energy Carriers and Water-Splitters. Int. J. Hydrogen Energy 1998;23:767-774.
[5] Steinfeld, A. Solar Hydrogen Production via a Two-Step Water-Splitting Thermochemical Cycle Based on Zn/ZnO Redox Reactions. Int. J. Hydrogen Energy 2002;27:611-619.
[6] Palumbo R, Lédé J, Boutin O, Elorza Ricart E, Steinfeld A, Moeller S, Weidenkaff A, Fletcher EA, Bielicki J. The production of Zn from ZnO in a single step high temperature solar decomposition process. Chem. Eng. Sci. 1998;53:2503-2518.
[7] Moeller S, Palumbo R. Solar thermal decomposition kinetics of ZnO in the temperature range 1950-2400 K. Chem. Eng. Sci. 2001;56:4505-4515.
[8] Weidenkaff A, Reller A, Wokaun A, Steinfeld A. Thermogravimetric analysis of the ZnO/Zn water splitting cycle. Thermochimica Acta 2000;359:69-75.
[9] Weidenkaff A, Reller A, Sibieude F, Wokaun A, Steinfeld A. Experimental investigations on the crystallization of zinc by direct irradiation of zinc oxide in a solar furnace. Chemistry of Materials 2000;12:2175-2181.
[10] Elorza-Ricart E, Martin PY, Ferrer M, and Lédé J. Direct thermal splitting of ZnO followed by a quench. Experimental measurements of mass balances. J. Phys. IV France 1999;9:325-330.
[11] Moeller S, Palumbo R. The development of a solar chemical reactor for the direct thermal dissociation of Zinc Oxide. J. Solar Energy Engineering 2001;123:83-90
[12] Haueter P, Moeller S, Palumbo R, Steinfeld A. The production of zinc by thermal dissociation of zinc oxide – solar chemical reactor design. Solar Energy 1999;67:161-167.
[13] Berman A, Epstein M. The Kinetics of Hydrogen production in the oxidation of liquid zinc with water vapor. Int. J. Hydrogen Energy 2000;25:957-967.
[14] Ly, H.C, Weiss, J.R., Wegner, K., Pratsinis, S.E., Steinfeld, A. Hydrogen production by the 2-step ZnO/Zn water-splitting thermochemical cycle ― Insitu formation and hydrolysis of zinc nanoparticles. Submitted to Int. J. Hydrogen Energy 2004.
[15] Weiss, J.R., Ly, H.C, Wegner, K., Pratsinis, S.E., Steinfeld, A. H2 Production by Zn Hydrolysis in a Hot-Wall Aerosol Reactor. Submitted to AIChE Journal 2004