Energy 2.0

The benefits of thermal storage systems for air-conditioning

Posted on: July 26, 2008

Thermal storage, introduced in building air-conditioning, is the system which levels the power load by storing the daytime power demand for air-conditioning, pushing the peak power demand up, in the form of heat in advance.

Energy-saving effects of thermal storage systems -Comparison with conventional air-conditioning systems-

(1) Improved efficiency of heat pumps using rated operation

In thermal storage systems, heat pumps always can be run at high-efficiency rated outputs without emulating the air-conditioning load variations.

This is because it is possible to control the amount of heat produced by increasing or decreasing operation hours of heat pumps not only during peak air-conditioning load periods but during light load periods.

(2) Improving efficiency of heat pumps using outside air temperature difference

As the outside temperature drops in night, heat pumps for storing heat become more efficient for cooling, but efficiency is reduced for heating.

For office buildings in which the annual cooling demand surpluses the annual heating demand largely, since the energy-saving effects for cooling excel, the efficiency of heat pumps operation improves over the year.

(3) Heat losses arising from the thermal storage tank

While the efficiency of heat pumps improves, heat losses occur from the thermal storage tank since there is a time lag until the chilled (or heated) water produced during the night is actually used for air-conditioning. Therefore, it is common to insulate inside the thermal storage tank for cutting down these heat losses and taking security of the energy-saving characteristics of thermal storage systems.

(4) Increase in carrying power due to thermal storage

In addition to requirement of two pumps in thermal storage systems, one is for thermal storage between heat pumps and thermal storage tank and another is for air-conditioning between storage tank and air-handling units, storage water is open to the atmosphere in many cases, the pump head and pumping power increases.

Energy efficiency comparison of thermal storage systems with other heat source air-conditioning systems

In air-conditioning systems for heating and cooling with absorption refrigerating machines and boilers as the heat generators using gas or oil rather than electricity for the heat source are widespread, particularly in the area of centralized air-conditioning. Recently also, co-generation system is becoming more widespread.

Recently in Japan there have been moves to promote the spread of co-generation which demonstrates overall efficiencies of 60 – 70%. However, particularly in commercial use, since annual load factor for heat demand is extremely low, approximately15-20%, while electric power demand such as lighting and motive power used firmly over the year, it is difficult to recover the exhaust heat as expected. Therefore there are many cases which can’t complete the overall efficiencies as the catalogue shows.

Even supposing the exhaust heat in co-generation is completely recovered, the energy efficiency in thermal storage system will be 10% better than that in co-generation, and in the future, with the introduction of the latest Advanced Combined Cycle power stations which have thermal efficiency of 50% at generating end, thermal storage systems will demonstrate 30% higher energy efficiencies compared with co-generation.

And comparing the amount of CO2 emission in “thermal storage systems at rated operation” with that in co-generation, thermal storage systems can reduce it for 40%.

As described above, it has been proven that thermal storage systems are extremely dominant as technology for efficient use of existing energy resources.

Therefore, it becomes important to devise the overall efficient use of energy resources with two wheels, one is the effort to improve thermal efficiency at power supply side and another is the high efficient use of energy by thermal storage system at demand side.

Shifting peak power demand by thermal storage systems also contribute to reduce the cost of electricity charges by controlling and cutting the cost for installing new power stations and to maintain mid/long-term energy security increasing the weight to nuclear power which carries a base load.

Benefits to users

The key to come thermal storage systems, which are superior in energy saving characteristics and the effect of power load leveling, into wide use depends entirely on the economic benefits for users.

It is said that the biggest problem of thermal storage systems are to increase the construction cost by installing thermal storage tank and complication of its design.

The benefits which improve this difficulty are as follows.

(1) Cost reduction of electricity charges

Our company has expanded the range of electricity charges, including discount schemes for nighttime electricity charges (1/3 – 1/4 of daytime charges). And in addition to this, we are actively tackling the promotion of widespread use through schemes such as the payment of fixed incentives to the manufacturers of ice storage systems.

Calculations of annual electricity charge benefits for a thermal storage system compared with a conventional system based on a typical office building, show economic benefits in a 53% reduction of electricity costs.

(2) Reductions in equipment size and cuts in machine room space

With thermal storage systems, size reductions can be achieved on substation, pipes in a machine room, and control equipment since the heat pumps are reduced in size or number, and this enables machine room space to be reduced in size. The provision of sufficient machine room space is a problem in buildings in the Tokyo area, so the reduction in space will be a major benefit to customers.

(3) Operational convenience

With thermal storage systems, sudden air-conditioning loads such as in the morning can be met by utilizing the thermal storage tank, but with conventional or other heat source air-conditioning systems, it is necessary to start up the heat source equipment to cope.

Thermal storage systems have also received high acclaim from operating staff for their ability to give peace of mind in air-conditioning operations since temporary back-up is possible with the chilled/heated water stored in the thermal storage tank even if an equipment failure should occur. By the way, the unmanned operation for thermal storage in the night can be achieved by the progress of control technology.

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