Maximum power point tracking converter based on the open-circuit voltage method for thermoelectric generators

Thermoelectric generators (TEGs) convert heat energy into electricity in a quantity dependant on the temperature difference across them and the electrical load applied. It is critical to track the optimum electrical operating point through the use of power electronic converters controlled by a Maximum Power Point Tracking (MPPT) algorithm. The MPPT method based on the opencircuit voltage is arguably the most suitable for the linear electrical characteristic of TEGs. This paper presents an innovative way to perform the open-circuit voltage measure during the pseudo-normal operation of the interfacing power electronic converter. The proposed MPPT technique is supported by theoretical analysis and used to control a synchronous buck-boost converter. The prototype MPPT converter is controlled by an inexpensive microcontroller, and a lead-acid battery is used to accumulate the harvested energy. Experimental results using commercial TEG devices prove that the converter accurately tracks the maximum power point during thermal transients. Precise measurements in steady state show that the converter finds the maximum power point with a tracking efficiency of 99.85%

Combined heat and power system for stoves with thermoelectric generators

Solid-fuel stoves are used in developing countries, remote locations, and in general more commonly due to convenient fuel cost for space heating. The possibility of also using the stove heat to heat water and produce electricity represents an added benefit. This work presents an application of thermoelectric generators to a solid-fuel stove to concurrently charge a lead-acid battery and transfer heat to water for heating or household use. The feasibility of the proposed CHP system is demonstrated for a common solid-fuel stove. This system produces an average of 600 Wth and 27 Wel (42 Wel peak) during a 2-h long experiment in which the TEG efficiency is around 5% and the MPPT efficiency of the power converters used is demonstrated

Rankine Cycle efficiency gain using thermoelectric heat pumps

The Rankine cycle remains the dominant method of thermal plant electricity generation in the world today. The cycle was described over 150 years ago and significant performance advances continue to be realised. On-going metallurgy research has enabled the operating pressure and temperature of the boiler and turbine to be increased, thereby improving the cycle efficiency. The ubiquitous use of the Rankine cycle on a massive scale in conjunction with fossil fuels as the energy source continues to motivate further efficiency improvements in the cycle.<p></p> Previous work established a theoretical basis for the use of thermoelectric heat pumps (THPs) in the condensation process of the Rankine cycle to positively impact cycle efficiency. The work presented here experimentally validates this prior work and provides performance metrics for current commercially available THPs and quantifies how their use can increase the efficiency of the Rankine cycle as implemented in a large power plant.<p></p> A commercial THP is characterised to obtain its Coefficient of Performance (COP) variation with input current and the amount of thermal energy transported. A larger-scale system comprising of a multistage thermoelectric heat pump is then considered, demonstrating that using commonly available THPs a fuel load reduction of over 1.5% is achievable for a typical generating set whilst simultaneously increasing the overall plant cycle efficiency from 44.9% to 45.05%.<p></p> The paper concludes with a cost-benefit analysis of the system, showing that over a four year period the saving in fuel used can easily re-coup the capital cost incurred by the addition of the condenser heat pump

The effect of temperature mismatch on thermoelectric generators electrically connected in series and parallel

The use of thermoelectric generators (TEGs) to recover useful energy from waste heat has increased rapidly in recent years with applications ranging from microwatts to kilowatts. Several thermoelectric modules can be connected in series and/or parallel (forming an array) to provide the required voltage and/or current. In most TEG systems the individual thermoelectric modules are subject to temperature mismatch due to operating conditions. Variability of the electro-thermal performance and mechanical clamping pressure of individual TEG modules are also sufficient to cause a significant mismatch. Consequently, when in operation each TEG in the array will have a different electrical operating point at which maximum energy can be extracted and problems of decreased power output arise.<p></p> This work analyses the impact of thermal imbalance on the power produced at module and system level in a TEG array. Experimental results clearly illustrate the issue and a theoretical model is presented to quantify the impact. The authors believe the experimental results presented in this paper are the first to validate a rigorous examination of the impact of mismatched operating temperatures on the power output of an array of thermoelectric generators

Constant heat characterisation and geometrical optimisation of thermoelectric generators

It is well known that for a thermoelectric generator (TEG) in thermal steady-state with constant temperature difference across it the maximum power point is found at half of the open-circuit voltage (or half of the short-circuit current). However, the effective thermal resistance of the TEG changes depending on the current drawn by the load in accordance with the parasitic Peltier effect. This article analyses the different case in which the input thermal power is constant and the temperature difference across the TEG varies depending on its effective thermal resistance. This situation occurs in most waste heat recovery applications because the available thermal power is at any time limited. The first part of this article presents the electrical characterisation of TEGs for constant-heat and it investigates the relationship between maximum power point and open-circuit voltage. The second part studies the maximum power that can be produced by TEGs with pellets (or legs) of different size and number, i.e. with different packing factors, and of different height. This work provides advice on the optimisation of the pellets geometrical parameters in order to increase the power generated, and consequently the thermodynamic efficiency, and to minimise the quantity of thermoelectric material used, for systems with limited input thermal power.</p

Thermoelectric energy harvester with a cold start of 0.6 °C

This paper presents the electrical and thermal design of a thermoelectric energy harvester power system and its characterisation. The energy harvester is powered by a single Thermoelectric Generator (TEG) of 449 couples connected via a power conditioning circuit to an embedded processor. The aim of the work presented in this paper is to experimentally confirm the lowest ΔT measured across the TEG (ΔTTEG) at which the embedded processor operates to allow for wireless communication. The results show that when a temperature difference of 0.6 °CΔTTEG is applied across the thermoelectric module, an input voltage of 23 mV is generated which is sufficient to activate the energy harvester in approximately 3 minutes. An experimental setup able to accurately maintain and measure very low temperatures is described and the electrical power generated by the TEG at these temperatures is also described. It was found that the energy harvester power system can deliver up to 30 mA of current at 2.2 V in 3ms pulses for over a second. This is sufficient for wireless broadcast, communication and powering of other sensor devices. The successful operation of the wireless harvester at such low temperature gradients offers many new application areas for the system, including those powered by environmental sources and body heat

Outcome predictors in rheumatoid arthritis

Predicting which patients will develop severe rheumatoid arthritis is essential for selection of the most appropriate treatment regimen in early arthritis. The key outcomes in rheumatoid arthritis are persistence of the disease, joint damage (evaluated by X-ray progression), functional dysability, and mortality rate. Rheumatoid factor positivity and number of swollen joints appear to be related to all of these outcomes, while radiologic scores are mostly related to joint damage and health assessment questionnaire (HAQ) to functional dysability. Other relevant prognostic parameters are erythrocyte sedimentation rate or serum C-reactive protein levels, and antibodies to citrullinated peptides

Multi-disciplinary robust design of variable speed wind turbines

This paper addresses the preliminary robust multi-disciplinary design of small wind turbines. The turbine to be designed is assumed to be connected to the grid by means of power electronic converters. The main input parameter is the yearly wind distribution at the selected site, and it is represented by means of a Weibull distribution. The objective function is the electrical energy delivered yearly to the grid. Aerodynamic and electrical characteristics are fully coupled and modelled by means of low- and medium-fidelity models. Uncertainty affecting the blade geometry is considered, and a multi-objective hybrid evolutionary algorithm code is used to maximise the mean value of the yearly energy production and minimise its variance

The seedbeds of Tuberculosis: is it time to target congregate settings and workplaces?

Countries where the incidence of Tuberculosis (TB) is low display a low transmission rate in the general population, and this rate has progressively declined in recent decades; however, TB epidemiology has shown a shift of the disease burden from the general population to specific populations at higher risk, such as vulnerable individuals and hard-to-reach groups. In low-incidence countries, preventive and therapeutic strategies must therefore be geared towards targeted interventions in these populations, with the priority being to promptly identify and treat latent tuberculosis infection (LTBI) rather than manage infectious cases. One of the most complex challenges in this area is to identify population subgroups with increased incidence/prevalence of LTBI/TB. The aim of this study was to provide a concise overview of the main studies and available evidence concerning the epidemiology of TB and LTBI in non-healthcare congregate settings, with specific emphasis on studies conducted in occupational settings and workplaces. Recognizing settings at increased risk might contribute to eliminating TB in low-incidence countries, a challenge which requires tailored responses.Occupational and preventive medicine has a major role to play in directing ad hoc policies and programs of LTBI surveillance. If TB is to be eradicated, it is essential to contain the seedbeds of infection: indeed, as long as a large reservoir of infected subjects exists, new active TB cases may arise at any time