How do you significantly reduce the energy use of a restaurant?

How do you significantly reduce the energy use of a restaurant?

An integrated hot water and fresh air tempering system powered by a Temperzone MAGNUS MWS single pass heat-pump water heater was installed at KFC Rolleston, New Zealand.

This case study provides an assessment of the performance of the system for 12 months. The system achieved an annual reduction in electricity usage of 72.6 MWh when compared with a conventional electric resistance-based system, an estimated savings of $10,900. The HPWH achieved an annual COP of 3.4, corresponding to a reduction in electricity usage of 68%. The system reduced the contribution of water heating and fresh air tempering to the total store electricity consumption from 23% to 8%.

Background

An integrated water and fresh air tempering system was installed at the Rolleston store. The system combines a Temperzone MAGNUS 50kW single pass heat-pump water heater (HPWH) with a 500 L thermally zoned storage tank. Also connected to the system is a 31.5kW fresh air tempering (FAT) coil, embedded in the supply air duct.

Integrating the potable (tap) water heating with the FAT system allows the capital cost of the HPWH to be spread over two significant energy loads, and utilising the spare capacity of the water heater. A zoned hot water storage tank was incorporated into the design to ensure the priority for hot water at the tap was maintained.

The system is monitored using a Splash monitoring system, to determine an energy balance of the water heating system. From this energy balance, the water and FAT energy demands can be separated.

Power and monitoring equipment has also been installed, which measures the total electricity demand of the store, and separately, the electricity consumption of the water heating system. This enables the calculation of the energy savings delivered by the heat-pump water heater as referenced against an all-electric system, as well as a proportion of the total store electricity use.

The traditional installation this system replaces is an 18kW electric storage water heater tank, and a 31.5kW electric duct heater, operating in three stages. A store in Glenn Innes, Auckland was initially monitored, which provided hot water and fresh air tempering usage data as a daily as well as a seasonal profile. This data was used to determine the likely energy usage, and therefore electricity savings at the Rolleston store.

System Design

There are several design constraints factored into this system. These are:

• Kitchen hot water is required at a minimum of 65˚C at the tap. Washroom hot water is tempered to 55˚C.

• Hot water is mostly used in the kitchen to defrost chicken in large sinks. Each defrost requires approximately 300 L of hot water within 10 minutes. Defrost events are periodic during the day, based on how busy the store is.

• There is minimal hot water usage during much of the day, other than for defrosting chicken.

• Significant quantities of hot water is also used in the late evening for store washdown.

• The hot water production of the HPWH decreases with decreasing ambient temperatures.

The FAT energy demand increases with decreasing ambient temperatures.

To meet the simultaneous hot water demand and FAT demand under design conditions (1˚C) ambient temperatures would have required either a substantially larger storage tank, or a doubling in the capacity of the HPWH. Both of these options would have significantly increased the capital cost of the system. An alternative approach has been used instead, using a zoned hot water storage tank in conjunction with a single pass HPWH.

Features of the system include

• The HPWH (MWS500) produces 62˚C water in a “single pass” through the system. This water is drawn from the bottom of the tank, and is added to the top of the lower tank. The “real time” production from the MWS500 is available for “real time” hot water usage.

• The upper tank (40 L) allows for water preheated to 62˚C to be heated to 65 – 68˚C to meet Building Code requirements for kitchen hot water. The upper tank can provide a continuous supply of 65˚C hot water at an unmixed rate of 7 L/min.

• The FAT coil takes 60˚C water from the top of the lower tank, and tempers the incoming air to 18˚C. A variable speed controlled pump maintains the FAT to 18˚C.

• Three temperature sensors allow for the tank to be Zoned, based on the amount of hot water remaining in the tank.

• Zone A: The HPWH begins a reheat cycle. There is full FAT service.

• Zone B: The variable speed pump reduces in speed to minimum to restrict FAT service. The FAT supply air temperature reduces from 18˚C to approximately 13˚C

• Zone C: FAT service is temporarily halted, with all remaining hot water prioritised to the kitchen taps.

• The FAT Controller maintains the FAT Supply Air temperature to 18˚C, maintains the upper tank temperature to 65-68˚C, and provides freeze protection of the FAT coil.

Read the full article in the Industry Journal, October 2021 issue