Space Cooling  »  CHCP systems

The heat recovered from the exhaust, cooling circuit and or lubrication system can be used to provide space heating and sanitary hot water, as well as be used to drive a thermally driven chiller. Residential buildings are unlikely to require simultaneous space heating and cooling demand, except in large glass fronted apartment buildings, but sanitary hot water demand provides a year round base load to which space heating or cooling is added.

Potential combime mnation of CHP priovers with possible thermally driven cooling solutions

Small-scale and micro-CHP in the residential sector can have an electrical capacity as low as 1 kWe and thermal capacity as low as 3-5 kWth, which would make them suitable for small apartments or low-energy single family dwellings. Field trials have shown that micro-CHP systems are most economic where constant and high loads are available. They are, in general, more suited to commercial buildings than residential buildings. However, cost reductions and improvements in performance mean that they will potentially play a role in the residential sector in the future.

The key design considerations for CHCP systems are the sizing of the electrical prime mover and its thermal output, as well as the outlet temperature, as this needs to be sufficient to drive the thermally driven chiller. This can become a trade-off as this may have an impact on the electrical efficiency of the system. The significantly higher capital costs of CHCP systems compared to a conventional boiler and air conditioning system require that the systems be run much longer to make economic sense. Scaling closely to the minimum load can help meet this criteria, but will then still require conventional heating and cooling systems, whose installed costs won’t be significantly lower than the full0scale systems otherwise required, at least in single-family dwellings. The other key determinant of the competiveness of a CHCP system is the electricity/gas price ratio, as the higher this is, the greater the value of the electricity generated from the CHCP system.

The trend towards tighter building standards for the energy performance of new buildings will reduce space heating and cooling loads significantly and will probably undermine the economics of distributed CHCP. The key market for CHCP will therefore likely be in new multi-family dwellings and in inefficient existing dwellings with high space heating needs.

CHCP prime movers and system details
A distributed CHCP system is integrated into buildings electrical, space heating, service hot water and space cooling system. A CHCP system will consist of an electrical prime mover that generates electricity, with a heat recovery system, a thermally driven chiller and a heat rejection system. In addition to this, an auxiliary conventional boiler and air conditioning system maybe be incorporated either for back-up, or if the CHCP system isn’t designed to meet the full potential load for space heating and cooling, and sanitary hot water. The system will probably also include hot water storage. This can act as a buffer to ensure the CHP system runs optimally, similarly, a cold water storage system can also allow more flexible and economic operation of the system.

The prime mover can be a steam turbine, gas turbine, micro-turbines, internal combustion engine (ICE), stirling engine or fuel cell. Steam turbines are typically large and generally start at 500 kW in size. This means that they are unlikely to be commonly used in the residential sector. However, in large multi-family dwellings with significant heating and cooling loads they may be an interesting solution. ICE CHP systems are the most common in use today, but their use in the residential sector poses some challenges. Although they are a mature and highly reliable system, in residential applications their noise levels can be a problem, while local pollutant emissions (NOx, SOx and particulates) are relatively high.  Gas turbines and micro-turbines can run on gaseous fuels and often light petroleum distillates. They are mature, although micro-turbines are only just being deployed in any numbers, and when using gaseous fuels have very low local pollutant emissions. They are quiet and are available in a range of sizes that are suitable for all residential applications. Stirling engines are external combustion engines and have the advantage of being quiet and being able to utilise a wide variety of fuels. However, the challenge they face is their low electrical efficiency, which makes the economics challenging. However, if costs come down, they could become a good solution for the residential sector, as they are also small and light.

Thermally driven chillers and desiccant evaporative cooling
The use of thermally driven chillers (TDC) (absorption or adsorption) or desiccant evaporative cooling (DEC) driven by the heat provided by the CHP system to providing space cooling is not widely deployed in the residential sector. However, the components of the systems at the scale required for large multi-family dwellings are mature technologies. The application of such systems in single-family dwellings is still at the early deployment stage. Not all thermally driven chillers or DEC systems can be paired with all CHP prime movers. The key considerations, apart from the overall sizing and pairing of the CHP and TDC systems is the interconnection between the CHP system and the TDC. The key parameters are:

• The working pressure used in the interconnection
• The heat transfer medium
• The temperature levels
• The power matching of the two systems

The heat transfer medium isn’t usually an issue, as this is normally water and there are no specific technical concerns. Similarly, the pressure used isn’t a constraint given most hydraulic system designs. Designing the CHP system to be able to provide more heat than the TDC or multiple TDCs linked together (for efficient operation at part loads and redundancy) may require allows sanitary hot water or space heating needs to also be met simultaneously. Matching of temperatures is an important area of the system design, as the TDCs require a minimum driving temperature, while the CHP system ideally wants to achieve the lowest outlet temperature feasible to maximise electrical efficiency. Of course, given that TDCs and CHP prime moves can operate in a certain range with nominal efficiency penalties, the key consideration is to model the performance of the system under the expected operating conditions to verify that the minimum outlet temperature of the CHP system doesn’t fall below the minimum required temperature of the TDC.