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PV pumped solar hot water heating systems. Photovoltaic solar panels + solar thermal energy. Solar P.V. module power & solar thermal collector. Solar controller operation. RHI solar heater control technology. Academic paper for Eurosun conference.

Solar thermal PV power controller. The Solartwin Solar Thermal Differential Controller: Overview.

Solar controller for PV pumped solar thermal systems: Introduction.

How is Solartwin controlled? This device is a zero carbon differential solar controller designed in-house exclusively for solar water heating systems which are powered by solar electricity. The solar controller monitors three temperatures, solar panel, top of cylinder and bottom of cylinder, and allows the solar pump to work when there is useful heat in the panel that can be stored. In the sunniest summer days, when little water is being used, the controller can also export excess heat at the end of the day to prevent the cylinder overheating from the sun’s rays.
Solar Controller

Solar controller: How It Works

The Solartwin solar thermal controller uses no mains electricity, as it is powered by the sun shining on a photovoltaic (PV) panel. It then stores the energy overnight in supercapacitors, which release energy constantly over 24 hours. Unlike rechargeable batteries, supercapacitors far better. They are are designed for 500,000 charge-discharge cycles, so your solar thermal controller is designed to be maintenance free over the life of your panels. The solar controller itself usually consumes less than 0.1 Watts of energy during normal use, so the balance, ie up to 5 Watts, is available to pump your water through the solar panel.

What Should The Customer Do ?

For a fully installed system, you do not need to do anything. The solar controller will be fully installed for you and sited at your convenience (usually in an airing cupboard right next to the cylinder). The temperature read-out on the front of the controller is the temperature at the top of the cylinder. You will also see a picture of a solar panel in the top left hand corner (as shown above). When it has arrows pointing inwards, then the panel is capable of saving you energy, and the pump should start once the controller is charged. In other words, it pumps when the panel temperature is hotter than the water in the bottom of your hot water cylinder. On the less common occasions when the arrows point outwards, this shows that the solar controller is trying to cool the cylinder to its normal maximum operating temperature. This is for your safety during extremely sunny conditions. In the extremely unlikely condition that a spanner symbol appears in the top right, this suggests a fault. Then you should call Solar Twin immediately on 0344 567 9032.

We hope you enjoy using the Solartwin Zero Carbon Solar Controller and appreciate watching
the system save you money.

Below is the text of a paper on the Solartwin solar thermal system pump controller which was presented at the Eurosun Conferences in Lisbon, Portugal, on October 2008.

The Zero Carbon Solar Thermal Solar Controller

Barry Johnston BSC, MSC, PGCE-FE1* and Simon Sharp2
1Solar Twin Ltd., 50 Watergate Street, Chester, Cheshire, CH1 2LA, United Kingdom, Tel: +44 1244 403 404
2 Solar Twin, Ageito, 4940-681 Rubiães, Paredes de Coura, Portugal, Tel: +351 913 759 291

Corresponding Author, barry (at) solartwin.com

Abstract (ES0833 The Zero Carbon Solar Thermal Solar Powered Controller)
The zero carbon solar thermal solar controller is an innovation from Solar Twin enabling solar water heating to get rid of its Achilles’ heel: the significant CO2 waste associated with using mains electric powered pumps for fluid circulation in solar panels. This invention is a photovoltaic (PV) powered solar controller which switches a variable speed brushless DC pump on when the water in the panel is hot enough to collect. This innovation could boost carbon savings in solar water heating systems by typically around 20%. These systems offer greater sustainability for the solar thermal industry avoiding the potential energy claw-back on conventional solar thermal. Solar water heating can now go green.

Keywords: Solar water heating, solar controller, zero carbon, solartwin.

1. Introduction

Solartwin is an innovative solar thermal system designed for solar water heating, utilising a number of unique design features, PV pumping, flexible silicone rubber pipework, a solar powered solar controller and extensive use of polymers. A major concern of the renewable energy industry as a whole relates to carbon footprinting of the manufacture, installation and operation of renewable energy technologies.

Focussing on the operational carbon factors (carbon clawback) and the need to minimise it or even to removing it altogether has been a major challenge to the solar thermal industry. Regarding solar thermal systems, a UK government funded study1 of eight domestic scale solar water heating systems confirmed what had long been suspected: that the environmental benefits of solar can be substantially improved by eliminating mains electricity. In this report, flat plate solar water heating systems negated an average of 17% of their potential global warming benefits (i.e. CO2 savings) by using mains electricity. For evacuated tubes, their loss averaged even higher, typically 23%. In other words, running a mains powered solar for ten years, its electricity consumption can negate its CO2 saving by around 2 years.

However, while much of Europe uses mains pumped solar thermal systems, even at present, three relatively well known forms of solar thermal systems have zero carbon clawback. These are: systems where the stored volume of water forms part of the collector itself, thus negating any need for pumping, and two further systems where the collector is separated from water store. In the first of these, where the solar panel is positioned below the store, thermosystem pumping, due to hotter water being less dense, can occur. In the second, the panel may be above the water store and pumping is carried out wholly by photovoltaics. It is this final option which is considered in this paper.

A recent life cycle analysis paper of three microgeneration technologies2 wind, PV and solar thermal, found that zero carbon solar hot water paid back the energy used in production of the system sixteen times over an assumed 25 year life. Therefore as innovators in the renewable energy sector, we felt the adoption of zero carbon solar hot water was an issue which needed to be tackled, not specifically for the Solar twin system since that was already PV pumped and zero carbon in operation, but for the industry as a whole. With improvements in operational carbon footprint in mind, we have developed a differential temperature controller which can be used for controlling a variety of photovoltaic (or low voltage DC) solar heating systems, including the Solartwin system.

Table 1. Solar Thermal system electric consumption comparison. A gas-electric carbon weighting figure of 2.5 was used in the paper. (Data taken from UK government funded study1 of eight solar water heating systems http://www.berr.gov.uk/files/file16826.pdf)

2. Development

Our primary aim in the development of this innovation was to zero carbonise the majority of standard domestic solar thermal systems. We considered the various elements to make this a possibility and assessed that the following criteria must apply to our zero carbon solar thermal solar controller. It should:

  • Be PV powered only (though with an option for low power DC)
  • Be flexible enough to work with a wide range of PV’s and DC pumps
  • Have a variety of easily chosen standard software programs onboard to accommodate a range of solar thermal systems and allow further engineer based customisation within the standard operational programs

We were also certain that the controller must be consumer rather than engineer orientated which meant that it must:

  • Be inherently safe and easy to fit for both professionals and those installing DIY domestic solar panels for home use
  • Be simple and tamper-proof, for example by having no interface buttons on the front of the unit meaning that it cannot be programmed or deprogrammed without first unscrewing the unit.
  • Have minimal need for component replacement by eliminating both batteries and mechanical relays. (Most rechargeable batteries are only capable of 200 – 1000 charge / discharge cycles, while relays end to degrade at physical contact points.)

Other elements vital to the design and optimum operation of the controller were felt to be:

  • That it should use very little power in operation, since PV power is costly
  • Use an integral microprocessor
  • Offer simple diagnostic display of situations where the controller is not working correctly (sensor disconnection, for example)
  • That the controller should be delivered pre-programmed

In developing the system we felt it important to design a schedule of development ensuring that we met both the needs of the consumer and the relevant regulatory bodies. Our initial perspective once we had identified a gap in the solar thermal market was to place a value on this gap and then identify regulatory issues for the relevant world markets. The next preparatory stage was to define boundaries making note of where boundary extensions were optimal.

Beginning product development, we defined several solutions to the issue of carbon claw-back in the solar thermal industry technically, both core and extensions. We further considered boundary issues and then on the basis of cost-benefits decided where they should be:

  • Included in version 1
  • Allowed for as a simple iteration of version 1
  • Would only be enterable as a remanufacture of version 1

On resolution of these issues we progressed to the development of hardware and software and to performing laboratory and in the field tests and then to iterate further product refinements.

Finally, we performed regulatory validation, finalised product documentation and brought version 1 to market in a predefined way, having previously completed market and competitor research.

3. Product Specification

We designed the system with ultra low power operation in mind with the controller designed from scratch, not as a modification to a mains powered controller. Pump switching is done using a thyristor rather than a relay in order to save energy.

One of our early design specifications was that the controller should not require battery replacement during a lifecycle in excess of 20 years operation. Our innovative controller therefore utilises an electricity store via super capacitors with 500,000 charge / discharge cycles (rechargeable batteries offer around 600 cycles only). The super capacitors are charged in the early morning daylight when the solar thermal system is most likely to be cool and there is insufficient power available to begin PV pumping around the system.

We also use day / night mode detection logic to reduce power consumption by increasing sensor interrogation time at night. The super capacitors offer over 30 hours display backup time and over 7 days program backup time without any power available. The controller will switch up to 1A DC and integrates internal self-resetting overload protection. It is suitable for operation with 18 and 36 cell PVs, nominally 11-21V open circuit voltage up to 20W with the facility to increase this if necessary. There is also an alternative option for low voltage DV power supply operation.

Our innovative controller uses an integral microprocessor together with three sensors (PT1000 sensor for panel) and a temperature display which can be top of cylinder temperature (default) or displays from all three sensors. The operation utilises both primary and secondary logic.

Primary logic offers:

  • Differential control
  • ON DT of 4-15C
  • OFF DT = from 2 degrees below ON DT to 2 degress
  • Pump off overrun time can be set between 0 seconds and 300 seconds

Whilst secondary logic offers:

What to do at high cylinder temperatures? – 3 options are
i. Pump on (always used with Solartwin)
ii. Pump off (for most conventional solar thermal)
iii. Pump remains differential (occasionally used with conventional solar thermal)
The temperature to implement secondary logic can be selected between 65 and 85C

The technical specification of the controller was a primary concern and with superior specification in mind we developed the unit with the following specification:

  • Temperature Range : -30C ~ +200C in panel and 0C to 105C on cylinder
  • Temperature Sampling Time : 30 Seconds by day but slower by night
  • Sensor types

i. TP Sensor : PT1000 ( -30C ~ +200C ), silicone coated 12m / 15m black or yellow coated
ii. TA Sensor : 103AT2 ( 0 C ~ +110C ), PVC wire coaxial black or red coated
iii. TB Sensor : 103AT2 ( 0 C ~ +110C ), PVC wire coaxial black ort blue coated blue

  • PV External Power 18 – 36 Cell PV, 5-30W, all within these total constraints, max 24V DC AND max 1.7A, whichever is lower.
  • Pump Control Output ON/OFF. (Variable speed comes from PV output).
  • Memory with Power Saving Design
  • Housing materials: Main box is ABS.
  • Programme input via 3 buttons which are inside the box
  • Energy storage 2 x 50F super-capacitor. (500,000 recharge cycles typical)
  • Switch is a mosfet max 60V 3,7A peak 25A
  • Self-resetting fuse max current is 1.5A-1.8A.
  • IC voltage tolerence 4-24V normal, or temporarily (under 1 second) at 40V

4. The Solartwin zero carbon solar thermal solar controller in use

This solar controller potentially allows most types of pumped solar thermal system to become zero carbon. As used with Solartwin, upstream of the controller is a PV panel and downstream of it is a pump with a brushless DC motor. The controller will provide differential pump control plus additional functionality. Energy is stored for the processor and display at night but not for the pump, although this can be added as an option. The controller will operate DC pumps up to 25W and 22V. On the Solartwin solar thermal system we use a 12V DC brushless DC motor on a diaphragm pump with an 18 Cell Crystalline PV.

Fig. 1. Simplified solar water heating from Solartwin

The design of the controller provides the end user with an attractive housing displaying temperatures, operation display and 5 cables: 2 for power and 3 sensors. The consumer is provided with onboard customisable aspects of program including:

  • temperature differentials
  • pump overrun time
  • choice of 3 overheat options: pump on, off and differential.

The overall design is user friendly, and suitable for DIY solar heating panels for home use, sensor cables are colour coded for example.

Fig. 2. The Solartwin zero carbon solar powered solar controller

When the system is used with a non Solartwin solar thermal system there may be small adjustments to be made on the temperature differentials when solar water heating incorporates a long pipe run (this is not as important with the Solartwin system because Solartwin utilises low volume flexible microbore silicone rubber piping rather than the larger copper pipes of traditional solar thermal). The following table displays the suggested adjustments.

TOTAL length (i.e. there  and back added together)  of unheated pipes (e.g. in  lofts and on roofs PLUS  HALF the length of heated  pipes (e.g. in the airing  cupboard and in heated  rooms)  Suggested  pump  overrun time  setting in  seconds.  Suggested  start difference  (over the bottom  of cylinder  temperature) in  degrees C  Suggested stop  difference (over  the bottom of  cylinder  temperature) in  degrees C

up to 10m 30-90 sec 4C 2C
10.1 to 15m 120 sec 6C 3C
15.1 to 20m 180 sec 8C 4C
20.1 to 25m 240 sec 10C 5C
25.1 to 30m 300 sec 12C 6C

Table 2. Temperature differential adjustment for longer pipe runs.

5. Benefits of zero carbon solar thermal control to the wider solar thermal industry

Our aim in developing this controller is to offer the solar thermal industry the opportunity to zero carbonise all solar thermal systems. It is becoming accepted that zero carbon design is the gold standard in European housing design. The UK government consultation document3 published in December 2006 and entitled, Building A Greener Future: TowardsZero Carbon Development, discusses this in some detail and states that, “developing new homes to low and zerocarbon standards on a large scale, we can promote technologies and innovation which will help drive down emissions from the existing stock too.

Our key goal is to achieve zero carbon new homes within a decade. Further, a recent life cycle analysis paper2 of three micro generators, states that “The UK domestic building sector, which contributes around 30% of final energy demand, and about 23% of greenhouse gas emissions, can play an important role in CO2 abatement. An uptake of low or zero carbon (LZC) distributed energy resources would help this sector to reduce fossil fuel energy use and CO2 emissions.

6. Conclusions
Zero Carbon PV powered controllers and pumping for solar thermal systems should be seen as the new gold standard. These solar energy systems offer greater sustainability for the solar thermal industry avoiding the potential energy claw-back of up to 33% on conventional solar thermal. The opportunity for solar thermal manufactures and their customers to fully embrace zero carbon solar thermal technology has now become a simple reality.

Since the technology is now available to achieve this step change, it can be argued that all new solar thermal systems in Europe should be PV pumped by an agreed target date (such as 2012) and the industry’s current somewhat atomised approach on component efficiency should soon be replaced by focussing on system sustainability. This should require zero carbon operation of solar thermal systems as mandatory in all domestic scale solar thermal installations.

EuroSun08 References
[1] Martin C, Watson M. Side by side testing of eight solar water heating systems. ETSU S/P3/00275/REP/2, DTI/Pub URN 01/1292
[2] Allen, S.R., G.P. Hammond, H. Harajili, C.I. Jones, M.C. McManus, and A.B. Winnett, 2008. Integrated appraisal of micro-generators: methods and applications. Proc. Micro-Cogen 2008, Ottowa, Canada, 29 April – 1 May, Paper MG2008-SG-005, 8pp
[3] Building A Greener Future: TowardsZero Carbon Development 07HC04711, report available on download on 1st July 2007 at the following location http://www.communities.gov.uk/publications/planningandbuilding/futuretowardszerocarbon

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