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