by Tom Martin
Both the governments of B.C. and Canada have set an ambitious target of 80 per cent greenhouse gas emissions (GHG) reductions by 2050, vs. 2007 levels and vs. 2005 levels respectively. Meeting these targets will require transitioning away from processes involving combustion of fossil fuels. For provinces with clean electrical grids, the implication for building heating systems is a transition towards heat-pumps as the primary means of heating with gas-fired boilers providing backup under near-peak load conditions.
While making the business case for retrofitting existing buildings with heat pumps can be a challenge due to the relative cost of natural gas and electricity (with gas being 2.5-3 times cheaper per unit energy in B.C.), the transition is already under way as governments and progressive organizations look to reduce the environmental impact of their facilities. Financially attractive projects generally involve recovering waste heat from existing cooling processes, modifying systems for heat recovery during end-of-life replacement projects, and recovering heat to low or medium temperature heating loads.
Large heat pumps and heat-recovery chillers typically use dual-stage centrifugal compressors or positive displacement compressors such as screw and scroll compressors. While centrifugal and screw compressors are generally used in large units with high fluid masses and one or two compressors, scroll compressors are very compact and favoured for modular heat pumps and chillers. Modular units generally have two scroll compressors and brazed plate heat exchangers with low fluid volumes and nominal cooling capacities of no more than 70-80 tons of cooling.
Modular heat pump or chiller plants involve combining many smaller units together in a parallel configuration and have a few key advantages over traditional plant design. Due to the number of compressors typically involved in a modular plant, there is a high level of redundancy since a single compressor failure may only reduce the plant capacity by a small percentage. From a retrofit perspective, the most significant advantage of modular heat pumps and chillers is their size. It will simply be much easier to move modular units into most existing mechanical spaces which can fit in service elevators and through typical double-doors.
As with all hydronic refrigeration equipment, ensuring that the equipment operates efficiently and facilitating maximum life expectancy requires maintaining stable operating conditions (heat exchanger temperatures and flows). Due to the low mass of fluid in the heat exchangers, reduction in fluid flows can result in rapid changes in temperature resulting in units shutting off (tripping) on high head pressure alarms. Modular heat pumps and chillers have something of a bad rap for being finicky, prone to failure and difficult to operate. The issues that these units have had in the field are generally due to problems with the hydronic system design.
Modular heat pumps should be designed to operate with constant flow through any given operating unit. To facilitate this, modular systems should always be designed in a primary-secondary configuration, with a dedicated circulation pump for the units and additional pumps for the system load distribution. The primary and secondary loops must be hydraulically separated such that varying conditions in the load distribution piping does not impact the pressure and flow in the primary loop. This separation can be achieved with the use of a bypass pipe or tank, sized for about two ft/s of fluid velocity through the bypass. Doing so minimizes the pressure drop of the bypass and ensures that the primary and secondary suction and discharge ports are all at equal pressures, eliminating any pressure interference between the loops.
The only means of heating or cooling capacity control for modular scroll units is through compressor cycling. The life expectancy of compressors is tied to the number of on/off cycles, so short-cycling (rapid on/off control) can significantly degrade compressor life and lead to motor burnout or a lubrication failure. Short cycling occurs when primary loop temperatures change rapidly, which can occur with low primary loop fluid volume. Adding a thermal buffer tank to the primary loop slows the rate of fluid recirculation and temperature change in the primary loop and allows the stored thermal energy to gradually discharge when the units cycle off. The required fluid volume depends on a number of factors but should generally be no less than 2.5 to three gallons/ton of system capacity.
Conveniently, both hydraulic separation and thermal buffering can be achieved with a single component – a fluid storage tank in a vertical configuration with four ports – two at a high-level and two at a low-level. The tank acts as the bypass pipe and adds the necessary fluid mass for thermal buffering. The vertical configuration is used to ensure temperature stratification. For a heating system, the primary discharge and secondary suction ports should be at a high-level where the heating fluid is hottest (and vice versa for a cooling system). Multiple tanks can be connected together if required to achieve the necessary fluid volume. The tanks can also be specified with built-in air and dirt separators.
If we are serious about achieving our current national and provincial GHG emissions reductions, then heat pumps are undoubtedly the future of building heating systems, with modular systems playing a significant role in larger retrofit applications. Following a few basic design principles will ensure these installations work effectively, resolving the perception of the systems as unreliable and expediting adoption of the technology.
Tom Martin is senior energy performance specialist, sustainability and energy, at WSP Canada Inc.