When designing hybrid heat pump systems, optimising the operational design conditions of both technologies and the hydronic design is essential to maximise heat pump contribution and system efficiency, says Ryan Kirkwood, Engineering Solutions Manager at Baxi.
As energy managers increasingly focus on reducing the carbon-intensity of the heat source in their buildings, low carbon heat pumps are one of the favoured solutions.
A fully served air source heat pump (ASHP) building will deliver one of the lowest carbon footprints in new build properties. Where an all-electric approach is not feasible, integrating ASHPs and high efficiency condensing boilers in a hybrid system can provide a more practicable solution to overcoming project restrictions while meeting heat demand more sustainability. When using mid-temperature heat pumps, our focus here, the challenge is to design both technologies into a harmonious design that successfully optimises individual and overall system efficiency.
Overcoming challenges
The first design consideration must be to maximise heat pump contribution within the project parameters.
Ensuring hydronic integration with peak or back up heat generation that does not penalise system performance or efficiency is also critical. However, this can present challenges due to the temperature differential of the two technologies. Temperature differential, also known as Delta T or ∆T, is the difference between the flow and return temperatures, or the temperature the heat pump or boiler puts out and the temperature which returns to it from the heating system.
Mid-temperature heat pumps typically work best at low flow temperatures and at a temperature differential of 5-10°C. Condensing boilers also operate more efficiently at lower temperatures but at a typical ∆T range of 10-40°C.
So what options are available to balance efficiency and technologies for optimal system performance?
Option 1: Load assist
Traditionally, a low carbon heat source runs lead in a baseload configuration with gas boiler(s) to assist as heat demand increases. While ASHP hybrid systems can also be configured this way, the primary flow temperature and subsequent temperature differentials must be suitable for both the ASHP and boiler technology.
Running a full system on a 10°C ∆T (leaving aside pipe and pump sizes) would not be problematic for most condensing boilers, but it can reduce the performance of some ASHPs which optimise at 5-7°C ∆T. To maximise boiler efficiency and promote condensing, the return temperature must be kept under 54°C.
One example of how this can be achieved is shown in Figure 1.
Figure 1
Two major advantages of this method are flexibility and scalability as both generators can run together or independently as demands fluctuate.
The ASHP charges the thermal store which then discharges, acting as lead boiler. The thermal store discharge pump can ramp up and down to match building load, avoiding full depletion as this would break the stratification layer and potentially give a lower flow temperature than set point.
Installing heat meters will provide valuable performance metrics to optimise the system and deliver live feedback to the BMS to help control the demand response.
The condensing boilers are cascaded on to duty assist with the demand.
Option 2: High Delta T load assist
In this next approach, the ASHP and boiler operate together in a system where the temperature differential or set point is above that which the ASHP can provide alone. A number of different methods can achieve this.
Thermal Store – As shown in Figure 2, the boiler and ASHP feed into the same thermal store with the energy required to satisfy system loads spilt. The ASHP maintains a stratified warm layer at the bottom of the tank to heat the cold return with the boiler’s hot layer providing the final lift to reach target flow temperature.
Figure 2
The controls should ensure that the boiler contribution is held back until absolutely required to avoid a temperature overshoot in the tank. Close monitoring of the tank temperatures at multiple points and of boiler and ASHP temperatures will help resolve this.
Injection method – Similar to the thermal store approach, this method uses the gas boiler to boost the flow temperature to set point at times when the ASHP is unable to satisfy the demand differential.
There are many variations on this method. Figure 3 illustrates an example of the injection method working on the return header prior to a low loss header.
Figure 3
The thermal store discharge pump controls the charge and discharge cycle of the thermal store. Under low load conditions, the store may be charged to system temperatures to give usable heat for a finite period without requiring the boilers to cascade on.
During periods of high demand, the thermal store injection will pre-heat the return for top up by the boilers. Ensuring proper discharge/charge cycling of the store is critical to the performance of this method.
Pre-heat efficiency loss, nett efficiency gain
We lose approximately 1-2.5% overall efficiency in a gas boiler when preheating a 30°C return by 5-10% due to slightly reduced condensing efficiency. This modest loss is massively countered by the renewable element of the ASHP efficiency. Provided the ASHP is not pre-heating the boiler return above condensing temperatures, the slight loss is acceptable to achieve a nett gain.
Avoiding conflict
In summary, along with standalone, purposed designed ASHP systems, hybrid solutions offer energy managers an opportunity for important efficiency gains and emission reduction. But to ensure optimal system efficiency and outcomes, careful consideration from the outset of flow and return temperatures, Delta Ts, controls and the detailed hydronic design will be crucial when blending the two technologies.