NOVELTY - The working fluid has several metal particles which are mixed in a heat conduction medium (H). The metal particles are provided with the alloy particles that are covered by a protection layer. The alloy particle is formed with the zinc and tin. The effective heat capacity of the working fluid is dependent on an expected ratio of the zinc to the tin. The temperature range of the latent heat absorption/releasing is dependent on the expected ratio of the zinc to the tin. The weight percent of the metal particles added to the heat conduction medium ranges between 1% and 20%.    USE - Working fluid for solar thermal system.    ADVANTAGE - The working fluid with higher heat capacity is provided to enhance the heat storage of the solar thermal system. The manufacturing process of metal particles is simplified at lower cost, and the effective heat capacity of the working fluid is increased. The temperature range is enhanced dependent on the ratio of the zinc to the tin. The latent heat absorption/releasing is controlled at the certain temperature range, thus the adjustable endothermic plateaus is applied to enhance energy storage for the various industrial systems.    DETAILED DESCRIPTION - The expected ratio of the zinc to the tin is controlled. The melting point of the protection layer is higher than the melting points of the zinc and the tin. The protection layer is made from metal oxide or silicon oxide. An INDEPENDENT CLAIM is included for a manufacturing method of metal particle, involves adding zinc and tin into the alcoholic solvent to deposit the tin on the zinc to form an alloy basis particle. The alloy basis particle is provided with an expected ratio of the zinc to the tin. The alloy basis particle is added into an alcoholic solvent, to form a first solution. The first solution is heated, and a precursor is added to the first solution to form a second solution. The pH value of the second solution is adjusted to between 4 and 5. An annealing procedure is implemented to form the protection layer on the surface of the alloy basis particle. The precursor is formed with 3-Aminopropyl trimethoxysilane (APTMS), - Glycidoxypropyltrimethoxysilane (GPTMS), or tetraethyl orthosilicate (TEOS). The annealing temperature of the annealing procedure is set in the range between 200 degrees C and 300 degrees C, and the annealing duration is set in the range between one hour and two hours. The metal particles are mixed in the heat conduction medium, to form the working fluid. An adjustable controllable ratio is determined as the expected ratio before adding the zinc and the tin into the alcoholic solvent to deposit the tin on the zinc to form the alloy basis particle.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the solar thermal system.    Solar thermal system (1)    Light reflecting apparatus (11)    Heat exchanging apparatus (13)    Storing apparatuses (14,15)    Heat conduction medium (H)