Stefan Larsson: Controller integration - Solenergi.dk

CONTROLLER INTEGRATION
2001-01-09
Stefan Larsson
Vattenfall Utveckling
Alvkarleby laboratory
SE-814 26 Alvkarleby
Stefan.larsson@utveckling.vattenfall.se
Abstract
The author shows the present integration's status of different solar-related as well as non-solar
functions of a controller for solar heating systems, especially focusing on solar combisystems. The
review is based on the products currently available from the market place, including cost aspects. The
integration may be performed at different levels, e.g. through hardware, software, networking, and/or
by means of using common sensors. The connection to a PC as well as upcoming new features are
also discussed. Considering the hardware currently in use, it is concluded that industry does not yet
utilise the full controllers' potential they sell to the market.
Common features in solar controllers
In solar heating systems the basic controller option is the differential temperature controller. The
basic controller only controls the solar-loop pump, although one common optional feature is that the
maximum store temperature is controlled, thus acting as boiling protection for the system. As the
industry is developing more advanced integrated systems with both solar and other sources of heat
(such as gas and biofuels), the need for new functions and more advanced integrated controller
functions emerges on the market.
Different aspects of integration
It can be seen that the solar heating systems are becoming increasingly complex as the market grows.
In some advanced systems, motor-actuated valves are used to redirect the flow towards different loads
in the system. Heating systems using more than one store or more than one boiler are frequent in solar
combisystems.
The ability to control the auxiliary-heat source is an option that has some advantages in respect to
higher solar fraction and utilisation of the store volume. It has been shown that it is possible to
increase the solar fraction by controlling the auxiliary-heat source in such a manner that the
stratification is maintained at a near optimum. It also gives the possibility to prevent the auxiliary-heat
source from being activated simultaneously with solar-collector operation.
In many situations the controller can be used for indoor climate control. Depending on the heating
system properties a 2- or 3-way valve can be controlled by means of the same hardware as the solar
controller function. The flow temperature can be controlled by the same reference parameters as
common systems do (i.e. by measuring the outdoor-, room- and flow temperature). An example of an
optional strategy that increases the solar gain is to decrease the room temperature setting during sunny
days, thus preventing overheating resulting from passive solar gain through windows at daytime.
Integration vs. networking of controllers
By networking controllers (i.e. multiple controllers share a communication bus in a network), the
integration occurs at the software level. One does not need to integrate the controller functions
physically into the same box. The key feature here is the logical function maintained in the software,

thus making the system work as one single unit. The controller function can be divided into two or
more separate parts, for example a solar-loop pump with internal controller operating networked with
a boiler controller. This is common technology in large district heating systems but not in small
domestic heating systems.
Another issue in networking controller systems is the sensor. In this line of development, the industry
is manufacturing intelligent sensors that can communicate over a simple serial bus network. One
example is the I-Button from Dallas Semiconductor. These sensors use a communication protocol that
only requires one single wire, and many sensors can share the same bus.
Wireless sensors are also becoming more and more interesting as the cost decreases for local
communication networks (i.e. inside the building or close to the building), but the cost is too high for
most solar applications today.
Remote operation of integrated controllers
By means of new technology in information distribution, it is possible to control a solar heating
system from remote locations. This makes possible to transfer weather data to the system with the aim
of turning off the auxiliary heater and loading a reduced amount of auxiliary heat into the store if
solar irradiation is predicted to be high. Such energy-efficient features have been demonstrated in the
non-solar industry for climate control of buildings.
User interface
The only most costly part of the controller is neither the processor nor the sensor A/D converter, but
the user interface. The user interface must give clear and stringent messages to the user and the
available buttons must have a clear function, in order to prevent misinterpretations and misuses. But
the high cost of the user interface forces the industry to use controllers with the simplest possible
means to communicate with the user. Often simplified controller interface is the major cause of
misinterpretations and user-caused malfunctions (the human factor).
The most common controllers on the market today have a simple 3-4 numerical digit LED display and
one push button or more for user input. By means of this interface, the user is able to monitor
temperatures and settings. By introducing additional LEDs, one can give the user additional
information in text labelled ”menus” to tell him which parameter is monitored or which function is
currently selected. This is a low-cost design but it also means that the final product is only able to do
its specific function and cannot be changed. One such example is showed in Figure 1.
Figure 1. An example of an integrated
controller for solar combisystems
(courtesy Lartec AB, Sweden)
At upper level, we see some more
advanced systems that have alphanumerical
LCD displays able to give more
information ”in clear speech” to the user.
This increases the ability to understand the
information provided. By using a rotary
knob instead of push buttons, the user can
easily navigate through the different
monitored values or settings. These designs
are more flexible and only require a new programme or ”operating system” to change functions.

The most-advanced controller on the market today uses a graphic LCD display that has the ability to
present diagrams and graphic elements to the user. Examples have been shown in which the user
interface is presenting ”icons” on the display like a PC environment that the user can navigate
through. It is also much easier to read a calendar function when it can be presented in a graphic
layout.
Today, more and more households have a PC. The establishment of a connection between a solar
controller and the PC in respect to the user interface is a powerful tool because more information can
be presented to the user. Most micro-controllers may be directly connected to a PC by using the serial
RS232 port. Most today's PCs also have high tolerances in the required voltage levels so that there is
no need for special RS232 voltage level interfaces or buffer circuits. Some examples of this feature
have also been shown by the industry.
Upcoming new features in integrated controllers
In recent years, some steps have been taken to use the controller for monitoring the solar gain. One
parameter easy to measure is the solar-loop pump operating time. Knowing the specific collector area,
the operating time and an estimate of how many hours the pump should run at the location considered,
the user can check if the system is working properly.
More-advanced options could also be economically feasible today. By measuring the solar-loop flow
rate, inlet/outlet and ambient temperatures, the hemispherical solar irradiance and the auxiliary energy
supplied to the system, it is possible to calculate the efficiency during normal operation even in small
domestic solar heating systems. Such features should increase the user’s ability to monitor the overall
system efficiency and should also increase the quality of the systems currently available from the
market place.
The implementation of these new features is possible in some of the most common micro-controller
processors used today by the industry. The main obstacle, however, is the implementation of
advanced mathematical functions. The standard 8- to 14-bit RISC processor core used by Atmel,
Microchip, Motorola, SGS-Thomson can only handle integer mathematics in small numbers up to 16-
bit binary, i.e. numbers between 0 and 65535 in decimal. It is however possible to overcome such
restrictions by developing ”operational systems” that translate or ”add” up to 32-bit floating point
mathematical functions. By using such software strategy, mathematical models could be implemented
directly into the microprocessor core. Some examples of embedded micro-controllers that have a
software core to emulate ”floating point co-processor” have also been demonstrated. By implementing
such hardware, one can use the mainstream controller as a combined control and a measuring and
evaluation system with a co-processor hidden inside the box (i.e. the user does not need to be aware of
the embedded system). This co-processor costs about USD 20-30 up to 32-bit arithmetic.
As sensor technology is becoming less expensive, new features like solar-loop glycol condition
analysis are made available. With simple pH sensors, the condition of the heat transfer fluid can be
monitored. By using intelligent ”noses”, ultrasound or optical systems, one can analyse other
properties in the fluid but at the penalty of higher system cost.
At all, this also means that the industry does not utilise the full potential of the controllers they sell to
the market, considering the hardware currently in use.