Project Specification

Horizontal Electric Water Heaters: A Characterization Platform

Platform characterize temperature variation and stratification of horizontal water heater

Specifications

Tank Volume150L
Custom Climatic Regulation Chambertwo 500-W air heaters; circulation fans; venting fans
Ambient Temperatures (Emulation)50 C
Inlet Water Temperature Regulationservo-controlled, three-port mixer valve
Outlet Water Temperature RegulationPI (proportional/integral) control system
Volumetric Flow Sensor Resolution+/-2.5 mL; 2.5 mL/pulse
associated error: 2.5 mL/s
Water Pressure 100 kPa
via standard pressure regulator valve
Thermostat - Hardware Cut-Off Temperature90 C
Mid-Region Of The Tank - Temperature Limit5%
5%Model: DS18B20 sensors
1-Wire communication protocol
Measuring range: -55 C to 125 C
Rated measurement error: +/-0.5 C
Sensor Chip Tubessilicon-filled, stainless steel tube
Diameter: 6 mm
Length: 50 mm
Support Framerectangular aluminum tubes
Thermal conductivity: 239 W/mK
Specific heat capacity: 900 J/kgK

Overview

Problem / Solution

Electric water heaters, with their thermal energy storage capacity, suit grid applications like demand management strategies well. Balancing various device aspects, like power loading, thermal losses, and user convenience, requires detailed modeling of the internal thermal dynamics of the tank, which also includes stratification. This depends on environmental factors, such as ambient and inlet temperatures, water draw patterns, and scheduling. Even the vessel orientation affects the stratification and temperature variation within the tank, bearing weight in the balancing act of demand management. While assessment of vertically oriented tanks has become possible, horizontal variety parameters are left to the guesswork, especially in developing countries.

Development on the embedded hardware and software platform involving temperature variations presents characterization inside horizontal water heaters, involving various environmental and usage conditions. Preliminary results show expected variations along the vertical axis and longitudinal and transverse axes under static and dynamic conditions. The interesting phenomena observed highlight the potential for a more extensive research study.

Design

The climatic chamber, emulating and controlling the ambient temperature, allows characterizing the EWH thermal behavior. A controllable in-line water heat exchanger likewise allows emulating the inlet water temperature. The user behavior is also observed by controlling usage patterns and the heating element's switching frequencies and conditions. Other significant goals include internal thermal stratification data acquisition, environmental and outlet water temperatures, and energy and water usage.

Environmental Emulation

Dynamic factors such as ambient and inlet water temperature make up the environmental conditions affecting the EWH thermal behavior and energy usage patterns. A custom climatic regulation chamber controls the ambient temperature using two 500W air heaters with circulation and venting fans. The actuators work in coordination, regulating a customized chamber temperature of up to 50 C.

The servo-controlled mixer valve regulates the inlet water temperature. One of the three inlet ports connects to a separate 100L EWH device inside a standard chest freezer. Removing the tank insulation also allows the decrease of resistance between the chest freezer cavity and the water. The second inlet port connects to the municipal water supply, and a PI control system regulates the outlet port temperature of the mixer valve.

User Emulation

As the usage pattern influences the behavior of EHW, generating more water usage profiles with respective energy responses allows for improved accuracy to that of existing models. Regulating parameters like volume, flow rates, and frequencies pave the way for such an improvement. Adding two sensors, the digital flow meter produces a digital pulse respective to the volume of water passing, and the outlet temperature sensor further improves the desired level of performance. The interrupt service routine (ISR) also helps accumulate the pulses of the flow meter. The per second accumulation of the sample uses a volumetric flow resolution of +/-2.5 mL, 2.5 mL/pulse with an error of 2.5 mL/s. The electric ball placed at the tank outlet ensures the EWH has enough pressure to regulate the water line when closed. Using a standard pressure regulator valve, the water pressure remains at 100kPa.

Electrical Utility and Thermostat Emulation

The design integrates an electronically controlled thermostat for electrical utility and thermostat emulation. A custom controller determines the state of the received temperature from the thermostat, basing the analysis on the defined temperature set in the software. Its inclusion provides dynamic set point changes, allowing for various heating control strategies.

As rolling blackouts may devoid EWH units of power for 4 hrs, electrical utility emulation plays a crucial role. Introducing a power availability schedule emulation, observing the effects of power interruptions on the EWH energy and stratification characteristics becomes possible. The controller now does two software checks, the power availability schedule that checks if power is available and temperature comparison to desired set temperature. This temperature checking can turn off when the sensed temperature is below the desired level, which also happens when the emulated utility checking detects no power. For additional safety in case of controller fault, the thermostat cut-off temperature is set to 90 C, ensuring the sensing region never surpasses this cut-off.

Data Acquisition and Sensor Selection

Aiding the thermal characterization of EWH in the horizontal axis is a vital pointer for experimentation. To consider this, the thermal stratification inside the tank, the temperature of various areas for energy analysis, and the usage and water flow rate are measured. The three-dimensional sensor orientation permits measuring the stratification across the horizontal tank in multiple areas.

The stratification measurement system placed inside the 150L tank shares space with the flange assembly installation, securing a well-positioned system inside. The tank initially positioned near one end is nearest to the heating element. Nine temperature sensors measure the stratification positioned on waterproof aluminum tubes. They are vertically placed through the tank's center and are spaced equally, allowing for a sufficient function of the sensors given their horizontal orientation.

The DS18B20 temperature sensor, with its convenient digital 1-wire communication protocol that supports multiple devices along similar data lines, provides accurate measurement range and resolution sensing. There are eight separate data lines for convenience in modularity and fault detection, arranged with fewer wires ideal for pressurized tanks. The sensor measures from -55 C to 125 C at an error of 0/- 0.5 C. Although in the range of 10-70 C, the error reduces to +/-0.2C.

Encapsulated in a silicon-filled stainless tube, made of 6mm-dia steel 50mm in length, the sensor chip is protected from rust accumulation—as its exposure to heat, and pressurized water extends for longer periods.

Structural Design Considerations and Suitability

With an aluminum support frame and a lightweight corrosion-resistant metal with suitable thermal properties, the stratification system can withstand thermal fluctuations and existing pressure. Aluminum has a high thermal conductivity of 239 W/mK compared to water's 0.598. It also has a specific heat capacity of 900 J/kgK, which is lower than water's 4200. These aluminum properties allow the tubes to lose and gain thermal energy faster than water, minimizing its impact on the thermal response of water.

Digital System Control

Arduino Due, an over-the-counter microcontroller, is a customized controller designed for the system to manage the experimental platform. It is also responsible for the various emulations (environment, user behavior, electrical utility) and thermostat control, likewise, the platform's data acquisition.

Every temperature bus module connects to the three-core silicone cable running to the outside of the tank. The existing anode rod has to be removed to provide an entry point. The cable and sensor buses are installed appropriately to provide spacing from the inlet water diffuser.

Sensor Referencing and Geometry Considerations

In sensor referencing, the recorded data from the test station are organized. There are 67 temperature sensors positioned at specific locations inside the tank. It also shows that the cross-section as a height function remains the same in vertically oriented tanks. The volume of water measured from every sensor node is the same when installed sensors are assumed of equal spacing. However, it differs for horizontally oriented tanks as the cross-section area in terms of height varies in a sinusoidal manner.

The central node on the vertical plane measures the largest volume of water against the top and bottom layers—influencing the average temperature along the plane. The calculation considers a weighted average since volume varies for each sensor. The nodal temperature is multiplied with the region volume over the total volume, acting as the contributor factor. To obtain the weighted average temperature, the 9 weighted values from the nodes are added.

References

A research paper describing the challenge, design, and outcome of the research.

Pieter D. van Schalkwyk, Jacobus A. A. Engelbrecht, and Marthinus J. Booysen

Wevolver 2022