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Renewable technologies are often lumped together under a single heading, yet the requirements of solar, biomass and heat pumps could not actually be more different. Many system designs at present are overlooking the essential differences between these technologies and, as a result, are failing to optimise the cost and carbon savings that are available.
Understanding the factors affecting efficiency of the various low and zero carbon technologies is the key to integrating them successfully. It can make the difference between a system in which renewables are there for show, and a system where renewables make real and quantifiable reductions in carbon footprint.
The three zero carbon technologies most commonly used in building services are solar thermal, biomass boilers and heat pumps.
Solar
There are three main types of solar thermal collector: flat plate, evacuated tube and unglazed plastic. Flat plate and evacuated tube collectors are most commonly used in the commercial environment.
Glazed flat plate collectors typically give 350 - 500kWh/m2. There are two types available of evacuated tube collectors- heat pipe and direct flow and these typically give 550-800kWh/m2.
Biomass
The most common form of biomass encountered in commercial building services applications is the direct combustion of wood in the form of chips or pellets.
Wood pellets are denser than wood chip (typically 600-700 kg/m3) and therefore require a much smaller storage area. They can also be blown or gravity fed and have an energy density of around 5kWh/kg. However, they are more expensive than wood chip and are generally less readily available. Typical boiler sizes range from 15kW to 150kW.
Wood chips are less expensive and less dense than pellets (typically 175-350kg/m3) and so require larger storage areas. The fuel feed system normally requires either a walking floor and/or auger feed and the energy density is around 2-4kWh/kg. However, wood chips are generally more readily available than pellets. Typical boiler sizes range from 25 to 1000kW.
Heat pumps
There are three generic types of heat pumps; ground source, air source and water source. Ground source heat pumps are classified as either water/water or water/air and are usually closed ground loops or an open loop. Closed ground loops are installed as either vertical bores, which are generally 100mm to 150mm in diameter and 15 to 180 metres deep and give around 1kW for every 30 metres, or horizontal Slinkys which are installed 1.5 to 2 metres deep.
Open loop systems utilise the heat available in ground water or ponds and lakes.
Air source heat pumps are also available. These usually have lower CoPs than ground source but do not require extensive civil work.
Operating temperatures
The renewable technologies are unlikely to be operating in isolation however. The zero and low carbon technologies are often relatively expensive in relation to the energy they provide, so current thinking often limits these solutions to 10 or 20% of the building load.
Integrating these technologies with one another, and with other high efficiency HVAC system components, is therefore crucial, and a major factor is the operating temperature that each technology requires in order to work at its most efficient. These are compared in Figure 1. From the chart it is clear to see that simply bolting on renewables can never work effectively given the differences in optimum operating temperatures.
Yes, the different technologies will function at other temperatures, but they will lead to maintenance issues, operational problems and/or reduced efficiencies.
So, if we design a system with a biomass boiler to take the base load with gas condensing boilers taking peak loads and providing standby back-up, then the former wants to run at 80/60°C whilst the latter wants to run at 50/30°C. In the vast majority of designs we see, the biomass camp wins because of the serious operational problems that would be the result of operating below 60°C. Thus the system is designed at best for 80/60°C or at worst for 80/70°C even though there are usually variable or low temperature circuits which require far less. Figure 2 shows a typical biomass/gas condensing boiler design.
This system will work. But it will cost more to install and it won't work at optimum efficiency. A 400kW system, designed at 80/70°C would cost around 19% more to install because of larger pipe and pump sizes and around 13% more in running costs than a truly integrated low and zero carbon system which operates each sub-system at its optimum operating temperatures.
A more effective alternative is the integrated design shown in Figure 3. In the solution shown here, the biomass boiler feeds a buffer vessel. On start-up the hot supply water is diverted via the 3-port valve into the biomass boiler return. Once the return water reaches 60°C, the 3-port valve starts to close to the by-pass and allows water through to the buffer vessel and eventually heats it to 80°C. The pump on the CT circuit draws water from the buffer vessel through the 3-port valve which is normally closed to the by-pass port.
The gas fired condensing boilers serve the VT circuit(s) with LTHW at 65/45°C. A 3-port mixing valve and pump on each VT circuit reduces the system temperatures further as required. Normally, the biomass and condensing boiler systems are separated by the 3-port diverting valve on the common flow header. However, in the event of a failure of either, the 3-port valve will open to allow flow from one to the other.
It is useful to note that it is not just with biomass and gas condensing boiler designs that these problems occur. It happens with all low and zero carbon systems like solar thermal and heat pumps as well.
System selection
The HVAC components to operate alongside the renewable technologies must, of course, be selected carefully. Traditional equipment typically does not have the required flexibility. But that doesn't mean that expensive bespoke kit is the answer. There are mainstream products readily available that are specifically designed for a part load, variable speed environment.
Variable speed plant (unlike conventional fixed speed plant) is always more efficient at part load. The latest generation of control methodologies can balance load to demand, ensuring the best wire-to-water efficiency at all times. That means that each system component is operated automatically, at its most efficient, to meet demand.
Particularly effective are HVAC components incorporating BMS compatible, application specific controls which adjust in real-time, with the criteria for energy efficiency pre-designed into the control methodologies. The renewable elements are then simply viewed by the system as additional contributing factors to the conditions, and the equipment adapts automatically around them.
Examples from Armstrong include the MBS integrated heating solution, IVS Sensorless variable speed drive pumps, Tenantherm thermal interface units, Quantum chiller, and the packaged systems built around our IPC Integrated Plant Controller.
Renewable technologies have moved from the fringe to the mainstream but, when designing systems, it is essential to ensure that their incorporation in a system involves significant improvement in carbon footprint, rather than necessitating a compromise.
