When using solar for the generation of domestic hot water, cylinders with twin coils are typically installed, but a new method for hot water generation integrates solar into a direct gas-fired water heater. Kevin Potter explains.
The traditional approach to using solar to generate domestic hot water (DHW), has been to install cylinders (calorifiers) with twin coils, where one coil is used to transfer the solar energy into the stored water, and the other to provide auxiliary heat input from another source, such as a condensing boiler.

However, a new approach for hot water generation integrates solar into a direct gas-fired water heater. The hybrid design combines a solar thermal hot water system with a high efficiency gas combustion system together in a single self-contained heating and storage vessel.

This compact solution, encompassed in a single water heater, takes up less space and can be installed close to the point of use, minimising any distribution heat losses and negating the need for an auxiliary heat source for topping-up heat or for anti-legionella cycling.

Separating the hot water generation from the space heating system in this way brings about a number of benefits, allowing the space heating system to be operated and maintained independently of the hot water system.

Featuring twin coil construction where one coil uses hot gases from a fully modulating down-firing gas burner designed for optimum condensing operation and efficiency, plus a second coil, rising from the bottom of the unit, which is heated hydraulically from the solar energy.

The gas burner in traditional atmospheric water heaters is located below the cylinder, and is susceptible to scale that drops to the base of the unit, creating an insulation barrier to the heat source.

This layer of scale can dramatically reduce the efficiency of the unit and lead to premature failure in the most extreme cases.

The solar water heater design with gas burner at the top, similar to modern condensing water heaters, eliminates this risk.

Integrated into the solar water heater is the intelligent control system that commands the water heater to maximise the solar contribution, taking advantage of nominally 'free' solar energy in place of gas heating whenever the collector temperature is just a few degrees above that of the stored water.

For every kilowatt of solar heat used in preference to gas heat (the solar coil has the potential to produce up to 30kW/h) there is a corresponding fuel cost saving, which equates to an equivalent reduction in CO2 emissions.

How it works
The solar and combustion heating circuits act in tandem to heat the stored water, with the solar circuit providing heat whenever collector temperature is just a few degrees (a programmable value, typically 5°C) above that of the stored water at the bottom of the tank, and with the gas circuit providing heat when the heating demand cannot be satisfied by solar heating alone.

When the stored hot water temperature falls below the combustion thermostat setting for gas burner operation, the controller makes an assessment of available solar energy.

If sufficient solar energy is available the burner operation will be held off allowing solar energy to heat the store. However, if the store temperature continues to fall, the gas burner will be brought into operation to heat the water.

The simplified curve in the graph shows typical temperature at the top of the tank against time, illustrating that the combustion circuit is held off from firing by hysteresis settings plus at least the programmed minimum solar differential.Thermostat and parameter settings can be adjusted, allowing the operation of the solar and gas heating cycles to be tuned to the specific installation.

The proposition of integrating solar into a direct fired water heater can be seen in the Dorchester DR-TC from Hamworthy.

The cylinder is designed for condensing performance and insulated with a layer of CFC-free foam to minimise standing heat losses.

With a gross efficiency of 96% the Dorchester DR-TC profits from all of the benefits associated with a solar thermal system such as lower fuel usage leading to reduced energy bills and lower carbon footprint, and yet still has a fast response time and impressive continuous output to satisfy demand all year round.

Further still, purchasers of the product can benefit from various government financial incentives, such as the Renewable Heat Incentive and are able to claim Enhanced Capital Allowances.

// The author is business development manager - hot water solutions, Hamworthy Heating //